Free ASCP MLS Practice Test: 60 Questions With Answers

Answer: B. Kidd antibodies are notorious for dropping below detectable levels months after the original immunization. A transfusion of Jk(a+) cells triggers a fast secondary (anamnestic) response, and the antibody returns days later and destroys the transfused cells — the classic delayed hemolytic transfusion reaction.

Why the others are wrong: A — clinically significant Kidd antibodies are usually IgG (sometimes with IgM) and react at 37 °C/AHG. C — proteolytic enzymes enhance Kidd reactivity; they destroy Duffy antigens. D — Kidd antigens are well expressed on adult red cells; the problem is antibody detectability, not antigen expression.

Source: PathologyOutlines, "Kidd blood group system" — Antibodies; Pathophysiology; Clinical features — https://www.pathologyoutlines.com/topic/transfusionmedkiddsystem.html; Dean L. Blood Groups and Red Cell Antigens (NCBI Bookshelf), "The Kidd blood group" — Clinical significance — https://www.ncbi.nlm.nih.gov/books/NBK2272/ (both checked Nov 1, 2026)

Answer: C. Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) = 138 − (98 + 12) = 28 mmol/L. A gap this high with a low bicarbonate points to an increased-anion-gap metabolic acidosis — think ketoacidosis, lactic acidosis, renal failure, or toxic alcohols.

Why the others are wrong: D adds potassium (138 + 4.5 − 110 = 32.5) — the question specified the formula without K⁺. A and B come from arithmetic slips (for example, subtracting only chloride and halving). Always check which formula your lab and the question use.

Source: StatPearls, "Biochemistry, Anion Gap" — Formula, Normal range and HAGMA sections — https://www.statpearls.com/point-of-care/17589 (checked Nov 1, 2026)

Answer: A. Corrected WBC = observed WBC × 100 ÷ (100 + nRBCs per 100 WBCs) = 15.0 × 100 ÷ 125 = 12.0 × 10³/µL. Use this correction when the reported WBC includes nucleated RBCs; do not apply it again if the analyzer has already reported an nRBC-corrected WBC. ASCP's content guideline lists WBC correction when more than 10 nRBCs are present among the calculations candidates are expected to know.

Why the others are wrong: B subtracts 25% of the count (15.0 × 0.75) — a common shortcut that gives the wrong denominator. C multiplies by 125/100, inflating the count. D ignores the nRBCs entirely.

Source: Nova Scotia Community College, Hematology Laboratory Manual, "WBC Correction for the Presence of NRBCs" — https://pressbooks.nscc.ca/mltd-hemlab/?p=346 (checked Nov 1, 2026); ASCP BOC MLS Content Guideline, Calculations section.

Answer: D. S. pneumoniae is inhibited by optochin and lyses in bile (sodium deoxycholate) because of its autolysin. Viridans streptococci are optochin resistant and bile insoluble.

Why the others are wrong: A describes S. pyogenes (group A). B describes S. agalactiae (group B). C describes Enterococcus.

Source: CDC job aid, "Alpha Streptococci vs. Streptococcus pneumoniae on Blood Agar" — https://reach.cdc.gov:443/sites/default/files/job-aids-resources/Alpha_Streptococci_vs_Streptococcus_pneumoniae_BAP_508.pdf (checked Nov 1, 2026)

Answer: C. Casts form in the renal tubules, so red cells trapped in a cast had to enter the nephron — almost always through a damaged glomerulus. RBC casts plus proteinuria after a strep infection is the textbook picture of post-streptococcal glomerulonephritis.

Why the others are wrong: A, B and D can all add free red cells to urine, but bleeding below the tubules cannot be packaged into casts.

Source: MSD Manual Professional, "Overview of Glomerular Disorders" (nephritic urine sediment) — https://www.msdmanuals.com/professional/genitourinary-disorders/glomerular-disorders/overview-of-glomerular-disorders (checked Nov 1, 2026)

Answer: A. Duffy antigens Fy^a and Fy^b are destroyed by proteolytic enzymes such as ficin and papain, so an antibody that reacts with untreated Fy(a+) cells and vanishes with enzyme-treated cells fits anti-Fy^a.

Why the others are wrong: B, C and D would each follow cells carrying their own antigen (Jk^a, E or K), not specifically the Fy(a+) cells. Kidd reactivity is also enhanced, not removed, by enzyme treatment, and Kell antigens are destroyed by thiol reagents such as DTT — a different reagent from ficin.

Source: Aldarweesh, "The Duffy Blood Group System" (IntechOpen), Section 2 — https://www.intechopen.com/chapters/69794; PathologyOutlines, "Kidd blood group system" — Laboratory — https://www.pathologyoutlines.com/topic/transfusionmedkiddsystem.html (both checked Nov 1, 2026)

Answer: A. Calculated osmolality = 2(140) + 90/18 + 14/2.8 = 280 + 5 + 5 = 290 mOsm/kg. Osmolal gap = 325 − 290 = 35 mOsm/kg. A gap this large means something osmotically active is present that the formula doesn't count — classically a toxic alcohol.

Why the others are wrong: B — a gap of 35 is not normal. C and D are arithmetic errors; dehydration raises both measured and calculated osmolality and doesn't create a gap.

Source: University of Iowa Hospitals & Clinics, Pathology Handbook, "Osmolality Gap — Calculation and Interpretation" — measured minus calculated osmolality; a large positive gap can indicate unmeasured osmoles including methanol and ethylene glycol — https://www.healthcare.uiowa.edu/path_handbook/appendix/chem/osmo_gap.html (checked Nov 1, 2026)

Answer: D. MCV = Hct (%) × 10 ÷ RBC (× 10⁶/µL) = 33 × 10 ÷ 5.50 = 60 fL (microcytic). Mentzer index = 60 ÷ 5.5 ≈ 10.9. An index below 13 favors thalassemia trait, where the marrow makes many small cells; above 13 favors iron deficiency. It's a screening clue only — iron studies and hemoglobin HPLC/electrophoresis confirm.

Why the others are wrong: A reverses the rule. B uses the wrong formula. C drops the ×10 factor.

Source: University of Alberta MLS Collection, "Red Blood Cell Indices, Colour, and Size" — RBC indices; microcytic RBCs — https://pressbooks.openeducationalberta.ca/mlsci/chapter/red-blood-cell-indices-colour-and-size/; Mentzer index interpretation (Mentzer WC, Lancet 1973;1:882) as summarized at https://calculate.qxmd.com/storage/calculators/878/Mentzer-Index.html (both checked Nov 1, 2026); ASCP BOC MLS Content Guideline, p. 14 (RBC indices).

Answer: B. Catalase-negative cocci that hydrolyze esculin in the presence of bile, tolerate 6.5% salt, and are PYR positive are enterococci.

Why the others are wrong: A — group B strep is PYR negative and doesn't grow in 6.5% NaCl. C — S. pyogenes is PYR positive but bile esculin negative and salt intolerant. D — staphylococci are catalase positive.

Source: MSD Manual Professional, "Enterococcal Infections" — Diagnosis — https://www.msdmanuals.com/professional/infectious-diseases/gram-positive-cocci/enterococcal-infections; NIOS DMLT Microbiology Lesson 17 (6.5% NaCl tolerance) — https://www.nios.ac.in/media/documents/dmlt/Microbiology/Lesson-17.pdf (both checked Nov 1, 2026)

Answer: D. Vaccination exposes the immune system only to surface antigen, so a vaccinated person makes anti-HBs but never anti-HBc. Anti-HBs alone = vaccine-induced immunity.

Why the others are wrong: A and B require a positive HBsAg. C — immunity from natural infection shows anti-HBc and anti-HBs.

Source: CDC, "Clinical Testing and Diagnosis for Hepatitis B" — serology interpretation — https://www.cdc.gov/hepatitis-b/hcp/diagnosis-testing/index.html (checked Nov 1, 2026)

Answer: C. Lewis antigens aren't made by red cells — they're adsorbed from plasma, and newborns have little or no Lewis antigen on their cells. Lewis antibodies are usually IgM, which doesn't cross the placenta. Together, those facts explain why Lewis antibodies don't cause HDFN.

Why the others are wrong: A — the premise is wrong. B — RhIG prevents anti-D, not Lewis antibodies. D — titer monitoring is for antibodies that can cause HDFN.

Source: Canadian Blood Services Professional Education, "Lewis system: anti-Lewis" — Lewis antibodies are predominantly IgM, do not readily cross the placenta, and fetal/neonatal red cells do not express Lewis antigens well — https://professionaleducation.blood.ca/en/transfusion/best-practices/serological-best-practices/lewis-system-anti-lewis (checked Nov 1, 2026)

Answer: D. LDL-C = TC − HDL-C − (TG ÷ 5) = 220 − 50 − 30 = 140 mg/dL. TG ÷ 5 estimates VLDL cholesterol in mg/dL. The equation is not valid when triglycerides exceed 400 mg/dL.

Why the others are wrong: A forgets the VLDL term. B divides TG by 2.5 instead of 5. C subtracts all of the triglyceride instead of TG ÷ 5.

Source: ADLM Clinical Chemistry Trainee Council, "Pearls of Laboratory Medicine: Routine Lipid Testing" (2012), slide 12 (Friedewald) — https://myadlm.org/Science-and-Research/Clinical-Chemistry/Clinical-Chemistry-Trainee-Council/Pearls-of-Laboratory-Medicine-in-English/2012/Routine-Lipid-Testing (checked Nov 1, 2026); ASCP BOC MLS Content Guideline, p. 14 (LDL/Friedewald).

Answer: A. Transferrin saturation = serum iron ÷ TIBC × 100 = 40 ÷ 450 × 100 ≈ 8.9%. Low iron, high TIBC, very low ferritin and low saturation is the iron-deficiency pattern — the body has made extra transferrin to grab scarce iron.

Why the others are wrong: B — chronic inflammation lowers iron but TIBC is low or normal and ferritin is normal or high. C — decimal error. D — thalassemia trait doesn't deplete iron stores; this ferritin rules that out as the sole explanation.

Source: Merck Manual Professional, "Iron Deficiency Anemia" — Diagnosis; Differential Diagnosis of Microcytic Anemia table — https://www.merckmanuals.com/professional/hematology-and-oncology/anemias-caused-by-deficient-erythropoiesis/iron-deficiency-anemia; AAFP, "Iron Deficiency Anemia: Evaluation and Management" (2013) — Diagnosis — https://www.aafp.org/pubs/afp/issues/2013/0115/p98.html (both checked Nov 1, 2026); ASCP BOC MLS Content Guideline, p. 14 (% transferrin saturation).

Answer: B. NAATs are the preferred diagnostic tests, but they can't tell you which antibiotics the organism resists. When treatment failure is suspected, culture is needed so the isolate can undergo susceptibility testing — and, in the U.S., so public health can track resistance.

Why the others are wrong: A detects nucleic acid only and gives no susceptibility data. C — Gram stain of cervical specimens has poor sensitivity and no susceptibility data. D — there's no useful serologic test for gonorrhea.

Source: CDC, Sexually Transmitted Infections Treatment Guidelines, 2021, "Gonococcal Infections Among Adolescents and Adults" — Diagnostic Considerations; Suspected Cephalosporin Treatment Failure — https://www.cdc.gov/std/treatment-guidelines/gonorrhea-adults.htm (checked Nov 1, 2026)

Answer: C. Two consecutive results for the same control exceeding the same 2 SD limit on the same side of the mean violates 2₂s. A consistent shift in one direction suggests systematic error such as a calibration drift or a deteriorating reagent.

Why the others are wrong: A — 1₂s is a warning, but here the pattern also meets a rejection rule. B — R₄s requires one control above +2 SD and another below −2 SD in the same run (a 4 SD range). D — no value exceeds 3 SD.

Source: Westgard QC, "Westgard Rules" — rule definitions — https://www.westgard.com/mltirule.htm; Basic QC Practices Lesson 18 — https://westgard.com/lessons/basic-qc-practices/lesson18.html (both checked Nov 1, 2026)

Answer: A. The pattern suggests a group A₂ person with anti-A₁ reacting with the A₁ reverse cells. Dolichos biflorus lectin agglutinates A₁ cells but not A₂ cells; a negative result with the patient's cells supports A₂ and helps explain the extra reverse reaction. It is interpreted with the rest of the ABO-discrepancy workup rather than used as a stand-alone confirmation.

Why the others are wrong: B — a DAT checks for in-vivo coating; it doesn't explain an unexpected reverse reaction. C — Ulex detects H antigen (useful for Bombay/weak subgroups), not A₁ vs A₂. D — prewarming helps with cold antibodies; the negative screen makes a cold alloantibody less likely, and anti-A₁ is the simpler explanation to test first.

Source: Dean L. Blood Groups and Red Cell Antigens (NCBI Bookshelf), "The ABO blood group" — A subgroups and lectin reactions — https://www.ncbi.nlm.nih.gov/books/NBK2267/ (checked Nov 1, 2026)

Answer: B. Indirect (unconjugated) bilirubin = total − direct = 3.2 − 0.3 = 2.9 mg/dL. A mostly unconjugated rise with high LD and low haptoglobin points to increased red cell destruction.

Why the others are wrong: A adds instead of subtracting. C — obstruction raises conjugated (direct) bilirubin. D reports the direct value as the answer.

Source: StatPearls, "Unconjugated Hyperbilirubinemia" — Etiology; Evaluation — https://www.ncbi.nlm.nih.gov/books/NBK549796/ (checked Nov 1, 2026); ASCP BOC MLS Content Guideline, p. 14 (unconjugated/indirect bilirubin).

Answer: C. Both B₁₂ and folate deficiency raise homocysteine, but only B₁₂ deficiency raises methylmalonic acid, because B₁₂ is the cofactor that converts methylmalonyl-CoA. Hypersegmented neutrophils confirm a megaloblastic process.

Why the others are wrong: A — folate deficiency leaves MMA normal. B — liver disease causes round macrocytes without hypersegmented neutrophils. D — iron deficiency is microcytic.

Source: StatPearls, "Vitamin B12 Deficiency" — Evaluation — https://www.ncbi.nlm.nih.gov/books/NBK441923/ (checked Nov 1, 2026)

Answer: D. H. influenzae needs both X factor (hemin) and V factor (NAD). S. aureus releases V factor and lyses red cells to free X factor, which produces "satellite" growth on sheep blood agar.

Why the others are wrong: A — H. parainfluenzae needs only V factor, so it would also grow around the V disk alone. B — Neisseria are gram-negative diplococci and don't need X or V factors. C — Moraxella are gram-negative diplococci, not coccobacilli, and don't need X or V factors.

Source: MicrobeCanvas, "Satellitism" — https://microbecanvas.com/tests/tests/satelliet-groei.html; WHO/CDC, Manual for the Laboratory Identification and Antimicrobial Susceptibility Testing of Bacterial Pathogens (2003), H. influenzae chapter — https://stacks.cdc.gov/view/cdc/11374 (both checked Nov 1, 2026)

Answer: B. The blood pad reacts with the peroxidase activity of hemoglobin and myoglobin. A strongly positive pad with almost no red cells, clear plasma, and a huge CK after muscle injury means myoglobin from rhabdomyolysis.

Why the others are wrong: A — ascorbic acid causes false-negative blood results. C — hemolysis would turn plasma pink (free hemoglobin is bound and cleared, haptoglobin falls). D — a specific gravity of 1.020 isn't dilute enough to lyse red cells.

Source: MSD Manual Professional, "Rhabdomyolysis" — Diagnosis — https://www.msdmanuals.com/professional/genitourinary-disorders/acute-kidney-injury/rhabdomyolysis; "Overview of Hemolytic Anemia" — Diagnosis — https://www.msdmanuals.com/professional/hematology-and-oncology/anemias-caused-by-hemolysis/overview-of-hemolytic-anemia (both checked Nov 1, 2026)

Answer: B. The labeled calculation uses the estimated volume of D-positive fetal red blood cells. Divide 30 mL by 15 mL per syringe: 30 ÷ 15 = 2 syringes. The label says to round a fractional result up to the next whole syringe; no extra syringe is added when the calculation is already a whole number.

Why the others are wrong: A covers only 15 mL of D-positive red cells. C and D exceed the labeled calculation for the stated 30-mL red-cell volume. In actual care, dosing follows the applicable product labeling, the measured fetomaternal hemorrhage, and local transfusion-service policy.

Source: FDA, HyperRHO Full Dose (Rho(D) Immune Globulin [Human]) package insert, Dosage and Administration: one 300-µg syringe covers 15 mL of D-positive red cells; divide calculated fetal red-cell volume by 15 mL and round fractional results up — https://www.fda.gov/media/191536/download?attachment= (checked Nov 1, 2026)

Answer: A. First convert volume to mL/min: 1,440 mL ÷ 1,440 min = 1 mL/min. Clearance = (U × V) ÷ P = (120 × 1) ÷ 1.2 = 100 mL/min.

Why the others are wrong: B and D skip the minutes conversion. C inverts a term.

Source: StatPearls, "Creatinine Clearance" — GFR approximation using creatinine clearance — https://www.ncbi.nlm.nih.gov/books/NBK544228/ (checked Nov 1, 2026); ASCP BOC MLS Content Guideline, p. 14 (timed urine and creatinine clearance calculations).

Answer: D. Absolute retic = % retic × RBC count = 0.040 × 2.50 × 10⁶/µL = 0.100 × 10⁶/µL = 100 × 10³/µL. That sits inside the ASCP composite range (20–115 × 10³/µL), which is a useful lesson: with an RBC count this low, a "normal" absolute count may still be an inadequate marrow response.

Why the others are wrong: A and B are decimal-place slips. C divides instead of multiplying.

Source: The Blood Project, "Reticulocytes — Knowledge Check" (absolute reticulocyte worked example) — https://www.thebloodproject.com/cases-archive/reticulocytes/knowledge-check (checked Nov 1, 2026); ASCP BOC MLS Content Guideline, pp. 14–15 (absolute counts; reticulocyte reference range).

Answer: A. Cefoxitin is a better inducer of the mecA gene product (PBP2a) and gives clearer results, so the cefoxitin disk is the recommended disk-diffusion surrogate for oxacillin resistance in S. aureus. Results are reported as oxacillin susceptible or resistant.

Why the others are wrong: B — oxacillin disk diffusion is not recommended for S. aureus because it performs less reliably. C — vancomycin disk diffusion is not reliable for staphylococci and doesn't detect MRSA. D — the penicillin zone-edge test detects beta-lactamase (penicillinase), not mecA.

Source: CDC, "Laboratory Testing for MRSA" — https://www.cdc.gov/mrsa/php/laboratories/index.html (checked Nov 1, 2026)

Answer: D. When the two screens disagree, a different treponemal test settles it. A reactive TP-PA supports past or present syphilis (clinical history decides whether treatment is needed); a nonreactive TP-PA suggests the first result was a false positive.

Why the others are wrong: A — you can't call it false positive without a second treponemal test. B — repeating the same assay doesn't resolve the conflict. C — CSF testing is for suspected neurosyphilis, not discordant screening.

Source: CDC, Sexually Transmitted Infections Treatment Guidelines, 2021, "Syphilis" — Diagnostic Considerations (reverse-sequence screening) — https://www.cdc.gov/std/treatment-guidelines/syphilis.htm (checked Nov 1, 2026)

Answer: B. RBCs are stored at 1–6 °C, but the federal transport requirement allows 1–10 °C during shipment. That distinction is a favorite exam trap.

Why the others are wrong: A is the storage range, not the shipping range. C is the platelet storage range. D is a frozen-plasma storage range.

Source: 21 CFR 600.15(a), shipping temperatures table — https://www.ecfr.gov/current/title-21/chapter-I/subchapter-F/part-600/subpart-B/section-600.15; 21 CFR 610.53(c), storage temperatures — https://www.law.cornell.edu/cfr/text/21/610.53 (both checked Nov 1, 2026)

Answer: C. The pH is low and the pCO₂ is high, so the primary problem is respiratory acidosis. Bicarbonate is still within the reference range (23–29 mmol/L) — in acute respiratory acidosis the kidneys haven't had time to retain much bicarbonate (only about 1 mmol/L per 10 mm Hg rise in pCO₂). Check: 6.1 + log(26 ÷ [0.03 × 60]) ≈ 7.26.

Why the others are wrong: A — metabolic acidosis would show low bicarbonate and low pCO₂. B — chronic compensation would raise bicarbonate further (about 3.5–4 mmol/L per 10 mm Hg) and move pH closer to normal. D — bicarbonate isn't high enough to signal a separate metabolic alkalosis.

Source: StatPearls, "Respiratory Acidosis" — Pathophysiology; Evaluation (acute vs chronic compensation) — https://www.ncbi.nlm.nih.gov/books/NBK482430/ (checked Nov 1, 2026); ASCP BOC MLS Content Guideline, p. 15 (arterial blood gas reference ranges).

Answer: B. Solubility tests need enough deoxygenated HbS to precipitate. Young infants still make mostly HbF, so the test can be falsely negative. It also can't distinguish sickle cell trait from disease — HPLC or electrophoresis is the right test.

Why the others are wrong: A — the test detects HbS (and some other sickling hemoglobins), not HbC. C — newborn screening by HPLC/electrophoresis diagnoses disease in infancy. D — dithionite reduction is exactly what makes HbS precipitate.

Source: Labcorp, "Hemoglobin (Hb) Solubility" — Limitations — https://www.labcorp.com/test-menu/27106/hemoglobin-hb-solubility; Brigham and Women's Hospital, "Sickle Cell Screening" — https://sickle.bwh.harvard.edu/screening.html (both checked Nov 1, 2026)

Answer: D. Erythromycin can switch on a resistance mechanism that also blocks clindamycin. The D-shaped zone shows that clindamycin resistance can emerge during therapy, so the isolate is reported clindamycin resistant.

Why the others are wrong: A — zone size alone isn't enough here; a round clindamycin zone (negative D-test) would support susceptibility, but this zone is blunted. B — a positive D-test changes the report; it doesn't call for a repeat. C — vancomycin is unrelated to the D-test.

Source: Hospital microbiology procedure, "D-Zone Testing for Staphylococci" (cites CLSI M100) — https://ltac.medtraining.org/ltac/docs/custom/202/24766_D-Zone Testing for Staphylococci.pdf (checked Nov 1, 2026)

Answer: C. Absorbance is directly proportional to concentration, so C(unknown) = (A(unknown) ÷ A(standard)) × C(standard) = (0.400 ÷ 0.250) × 100 = 160 mg/dL.

Why the others are wrong: A inverts the ratio. B and D are decimal-place errors.

Source: LibreTexts Chemistry, "The Beer-Lambert Law" — https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Electronic_Spectroscopy/Electronic_Spectroscopy_Basics/The_Beer-Lambert_Law (checked Nov 1, 2026); ASCP BOC MLS Content Guideline, p. 14 (Beer's law).

Answer: A. Cryoprecipitate concentrates fibrinogen (along with factor VIII, von Willebrand factor, factor XIII and fibronectin) into a small volume. Quality-control standards call for at least 150 mg of fibrinogen per unit, and federal rules require an average of at least 80 IU of factor VIII.

Why the others are wrong: B — the platelet count is adequate. C — RBCs restore oxygen-carrying capacity, not clotting factors. D — albumin contains no clotting factors.

Source: AABB, Standards for Blood Banks and Transfusion Services, 35th ed. (effective Apr 1, 2026), cryoprecipitated AHF quality-control requirements; FDA, 21 CFR 640.54 (Factor VIII content) and 21 CFR 606.122 (Circular statement that Cryoprecipitated AHF usually contains at least 150 mg fibrinogen) — https://www.aabb.org/standards-accreditation/standards; https://www.ecfr.gov/current/title-21/chapter-I/subchapter-F/part-640/subpart-F/section-640.54; https://www.ecfr.gov/current/title-21/chapter-I/subchapter-F/part-606/subpart-G/section-606.122 (checked Nov 1, 2026)

Answer: D. Red cells contain far more potassium than plasma. In-vitro hemolysis releases it and falsely raises the result (pseudohyperkalemia). The right fix is a properly collected new specimen.

Why the others are wrong: A reports a result you know is invalid. B — there's no reliable correction factor for hemolysis. C — rerunning the same hemolyzed specimen gives the same falsely high value.

Source: StatPearls, "Hyperkalemia" — Etiology (pseudohyperkalemia); Evaluation — https://www.ncbi.nlm.nih.gov/books/NBK470284/; ADLM Scientific Shorts, "Pseudohyperkalemia" (2019) — https://myadlm.org/Science-and-Research/Scientific-Shorts/2019/Pseudohyperkalemia (both checked Nov 1, 2026)

Answer: A. EDTA-dependent antibodies can make platelets clump in the tube, so the analyzer undercounts them (pseudothrombocytopenia). Recollecting in another anticoagulant, usually sodium citrate, and reviewing the smear gives the true count.

Why the others are wrong: B and D act on a spurious result. C — HIT is a real in-vivo thrombocytopenia; this is an in-vitro artifact.

Source: EDTA-dependent pseudothrombocytopenia case report and review, PMC11947505 — Discussion (diagnostic confirmation; blood smear) — https://pmc.ncbi.nlm.nih.gov/articles/PMC11947505/ (checked Nov 1, 2026)

Answer: C. C. difficile testing is meant for patients with diarrhea. Testing formed stool mostly detects colonization, which leads to overdiagnosis and unnecessary treatment.

Why the others are wrong: A, B and D are valid methods for an appropriate (unformed) specimen, but they shouldn't be run on formed stool.

Source: McDonald LC, et al. Clinical Practice Guidelines for Clostridium difficile Infection in Adults and Children: 2017 Update by IDSA and SHEA. Clin Infect Dis 2018;66(7):e1 — Recommendations VI–VII — https://academic.oup.com/cid/article/66/7/e1/4855916 (checked Nov 1, 2026)

Answer: A. Using ASCP's composite CSF ranges (WBC 0–5/µL, glucose 50–80 mg/dL, protein 15–45 mg/dL): a very high, neutrophil-predominant cell count, a very low glucose, and high protein is the bacterial pattern.

Why the others are wrong: B — viral meningitis usually shows lymphocytes and normal glucose. C — TB and fungal meningitis lower glucose but usually show lymphocytes. D — hemorrhage shows many red cells and xanthochromia.

Source: MSD Manual Professional, "Acute Bacterial Meningitis" — Cerebrospinal Fluid Findings in Meningitis — https://www.msdmanuals.com/professional/neurologic-disorders/meningitis/acute-bacterial-meningitis (checked Nov 1, 2026); ASCP BOC MLS Content Guideline, p. 16 (CSF reference ranges).

Answer: D. Hypertension, distended neck veins, a high natriuretic peptide, and a patient with heart failure all point to volume overload — TACO.

Why the others are wrong: A — TRALI also causes bilateral infiltrates, but by definition there's no evidence of left atrial hypertension (circulatory overload), and blood pressure often falls. B — anaphylaxis brings hypotension, bronchospasm and urticaria. C — FNHTR is fever and chills without respiratory failure.

Source: CDC NHSN, Biovigilance Component Hemovigilance Module Surveillance Protocol — Table 5 (TACO) and Table 6 (TRALI) — https://www.cdc.gov/nhsn/pdfs/biovigilance/bv-hv-protocol-current.pdf (checked Nov 1, 2026)

Answer: C. Cardiac troponin is the preferred biomarker. Acute myocardial injury requires a rise and/or fall in cardiac troponin with at least one value above the sex-specific 99th-percentile upper reference limit; with clinical evidence of acute myocardial ischemia, that pattern supports acute MI.

Why the others are wrong: A and B are less specific biomarkers and do not replace cardiac troponin criteria. D — a persistently elevated but stable troponin supports chronic myocardial injury rather than the dynamic injury pattern required for acute MI.

Source: Fifth Universal Definition of Myocardial Infarction (2026), JACC / European Heart Journal — cardiac troponin criteria for acute myocardial injury and MI — https://www.jacc.org/doi/full/10.1016/j.jacc.2026.07.025 (checked Nov 1, 2026)

Answer: A. This is the classic chronic myeloid leukemia smear. CML is defined by the Philadelphia chromosome, t(9;22), which creates the BCR::ABL1 fusion — detectable by cytogenetics, FISH or PCR.

Why the others are wrong: B defines acute promyelocytic leukemia. C is typical of polycythemia vera and some other BCR::ABL1-negative MPNs. D screens for paroxysmal nocturnal hemoglobinuria.

Source: StatPearls, "Chronic Myeloid Leukemia" — Introduction; Pathophysiology — https://www.ncbi.nlm.nih.gov/books/NBK531459/ (checked Nov 1, 2026)

Answer: B. Coagulase-negative staphylococci are common skin flora. Growth in a single set, with negative companion sets and no device, usually means contamination during collection.

Why the others are wrong: A and C — true CoNS bacteremia usually shows growth in multiple sets, typically in patients with lines or prosthetic material. D — the identification isn't the problem; the clinical significance is.

Source: AHRQ PSNet WebM&M, "Contaminated or Not? Guidelines for Interpretation of Positive Blood Cultures" — https://psnet.ahrq.gov/web-mm/contaminated-or-not-guidelines-interpretation-positive-blood-cultures (checked Nov 1, 2026)

Answer: D. Allergic contact dermatitis is a delayed, T-cell–mediated (type IV) reaction. The 24–72-hour delay is the clue.

Why the others are wrong: A — IgE-mediated reactions start within minutes. B — antibody against cell-surface antigens (for example, hemolytic transfusion reactions). C — immune-complex disease (for example, serum sickness, lupus nephritis).

Source: MSD Manual Professional, "Overview of Allergic and Atopic Disorders" — Type IV hypersensitivity — https://www.msdmanuals.com/professional/immunology-allergic-disorders/allergic-autoimmune-and-other-hypersensitivity-disorders/overview-of-allergic-and-atopic-disorders (checked Nov 1, 2026)

Answer: C. Related donors may share HLA haplotypes with the recipient, so the recipient's immune system may not recognize and reject the donor's lymphocytes — which can then attack the recipient. Irradiation inactivates donor lymphocytes.

Why the others are wrong: A and B reduce but don't reliably eliminate viable lymphocytes. D prevents CMV transmission, not TA-GVHD.

Source: AABB, "Irradiation" — cellular components from blood relatives are among risk settings for transfusion-associated graft-versus-host disease; irradiation prevents proliferation of donor T lymphocytes — https://www.aabb.org/regulatory-and-advocacy/regulatory-affairs/regulatory-for-blood/irradiation (checked Nov 1, 2026)

Answer: B. Digoxin distributes slowly from blood into tissue. Serum levels drawn soon after a dose are falsely high relative to the tissue concentration that matters, so levels are drawn at least 6–8 hours after dosing.

Why the others are wrong: A acts on a level that doesn't reflect steady-state tissue exposure. C and D have no supporting evidence in the scenario; timing is the obvious problem.

Source: University of Calgary Cumming School of Medicine, "CPT Pearl: Digoxin Monitoring" — https://cumming.ucalgary.ca/sites/default/files/teams/127/CPT Pearl Digoxin Monitoring.pdf; North Bristol NHS Trust Severn Pathology, "Digoxin" sample timing — https://www.nbt.nhs.uk/severn-pathology/requesting/test-information/digoxin (both checked Nov 1, 2026)

Answer: A. Acute promyelocytic leukemia is defined by t(15;17) creating PML::RARA. It's a laboratory emergency because of the high risk of DIC — and it responds to all-trans retinoic acid (ATRA).

Why the others are wrong: B — CML (and some ALL). C — myeloproliferative neoplasms. D — Burkitt lymphoma.

Source: StatPearls, "Acute Promyelocytic Leukemia" — Histopathology; Evaluation; Treatment — https://www.ncbi.nlm.nih.gov/books/NBK459352/ (checked Nov 1, 2026)

Answer: C. Nocardia are branching, beaded, gram-positive rods that hold the stain only when a weak decolorizer is used — they're "partially" (modified) acid-fast. They cause pulmonary and disseminated infection in immunocompromised patients.

Why the others are wrong: A — Actinomyces branch but aren't acid-fast. B — M. tuberculosis is strongly acid-fast with standard stains and doesn't branch. D — Corynebacterium are club-shaped and not acid-fast.

Source: MSD Manual Professional, "Nocardiosis" — Diagnosis — https://www.msdmanuals.com/professional/infectious-diseases/gram-positive-bacilli/nocardiosis (checked Nov 1, 2026)

Answer: D. PPV = TP ÷ (TP + FP) = 90 ÷ 110 = 81.8% — of everyone who tests positive, about 82% truly have the condition.

Why the others are wrong: A is sensitivity, TP ÷ (TP + FN) = 90 ÷ 100. B is specificity, TN ÷ (TN + FP) = 880 ÷ 900. C is negative predictive value, TN ÷ (TN + FN) = 880 ÷ 890.

Source: University of Pittsburgh Supercourse, 2×2 table definitions — https://sites.pitt.edu/~super1/lecture/lec0591/013.htm (checked Nov 1, 2026)

Answer: B. IgG coating on the red cells plus a panreactive eluate in a patient with no recent transfusion is the warm autoantibody pattern.

Why the others are wrong: A — cold agglutinin disease typically shows C3d only on the DAT. C — impossible without a recent transfusion, and the eluate would show a specific alloantibody. D — PCH (Donath-Landsteiner antibody) typically shows C3 on the DAT.

Source: University of Toronto LearnSerology, "Warm Reactive Autoantibodies — Confirmation: Eluate Testing" — https://pie.med.utoronto.ca/learnserology/LearnSerology_content/LS5_Warm_Reactive_Confirmation.html; Merck Manual Professional, "Autoimmune Hemolytic Anemia" — DAT patterns in warm AIHA, cold agglutinin disease and PCH — https://www.merckmanuals.com/professional/hematology-and-oncology/anemias-caused-by-hemolysis/autoimmune-hemolytic-anemia (both checked Nov 1, 2026)

Answer: B. Absorption and distribution are not complete until about 4 hours after ingestion, so earlier levels can't be interpreted on the nomogram.

Why the others are wrong: A and C are too early to interpret. D — the nomogram's line extends to 24 hours, but waiting that long to draw the first level would delay treatment that is most effective early.

Source: StatPearls, "Acetaminophen Toxicity" — Evaluation; Treatment/Management (Rumack-Matthew nomogram) — https://www.ncbi.nlm.nih.gov/books/NBK441917/ (checked Nov 1, 2026)

Answer: D. Normal plasma supplies the missing factor, so a deficiency corrects in the mix and stays corrected after incubation.

Why the others are wrong: A — a lupus anticoagulant typically fails to correct. B — factor VIII inhibitors are time-dependent; they may look corrected immediately but prolong again after incubation. C — heparin would also inhibit the normal plasma, so the mix wouldn't correct.

Source: Matsuda K, et al. Res Pract Thromb Haemost. 2023 (mixing test interpretation), PMC9986643 — Introduction — https://pmc.ncbi.nlm.nih.gov/articles/PMC9986643/ (checked Nov 1, 2026)

Answer: B. Crescent (banana-shaped) gametocytes are diagnostic of P. falciparum. Multiple rings per cell, appliqué forms, and a smear that usually shows only rings and gametocytes (later stages are seldom seen in peripheral blood) all fit. P. falciparum is also the species most likely to cause severe disease.

Why the others are wrong: A — P. vivax infects enlarged red cells with Schüffner's dots and shows all stages. C — P. malariae has band forms and low parasitemia. D — Babesia can show multiple rings and tetrads ("Maltese cross") but never produces crescent gametocytes.

Source: CDC DPDx, "Malaria" — P. falciparum morphology; comparison table — https://www.cdc.gov/dpdx/malaria/index.html (checked Nov 1, 2026)

Answer: B. Needle-shaped, strongly negatively birefringent crystals are monosodium urate — the diagnosis is gout.

Why the others are wrong: A — CPPD (pseudogout) crystals are rhomboid and weakly positively birefringent. C — cholesterol forms flat plates with notched corners. D — calcium oxalate is unusual in synovial fluid and isn't the classic needle-shaped crystal.

Source: MSD Manual Professional, "Gout" — Synovial fluid analysis; Microscopic Examination of Crystals in Joints — https://www.msdmanuals.com/professional/musculoskeletal-and-connective-tissue-disorders/crystal-induced-arthritides/gout (checked Nov 1, 2026)

Answer: C. Daratumumab binds CD38 on reagent red cells. DTT treatment eliminates the anti-CD38 interference, but DTT also destroys Kell antigens. AABB therefore recommends K-negative units unless the patient is known to be K-positive.

Why the others are wrong: A — autoadsorption doesn't remove a drug bound to all red cells. B — enzyme treatment isn't the established method for removing daratumumab interference; DTT is. D — skipping compatibility testing is unsafe and unnecessary.

Source: AABB Association Bulletin #16-02, "Mitigating the Anti-CD38 Interference with Serologic Testing," revised March 2026 — DTT-treated cells and K-negative units unless the patient is known to be K-positive — https://www.aabb.org/docs/default-source/default-document-library/resources/association-bulletins/ab16-02-revised.pdf (checked Nov 1, 2026)

Answer: B. When the thyroid itself overproduces hormone (for example, Graves disease), high free T4 feeds back on the pituitary and suppresses TSH.

Why the others are wrong: A — high TSH, low free T4. C — inappropriately normal or high TSH with high free T4. D — high TSH with a normal free T4.

Source: American Thyroid Association, "Thyroid Function Tests" — TSH and free T4 interpretation — https://www.thyroid.org/thyroid-function-tests/ (checked Nov 1, 2026)

Answer: A. Consumption of both platelets and clotting factors (long PT and aPTT, low fibrinogen) plus high D-dimer and schistocytes, triggered by sepsis, is DIC.

Why the others are wrong: B — TTP causes thrombocytopenia and schistocytes, but PT and aPTT are typically normal. C — ITP has isolated thrombocytopenia with normal coagulation tests and no schistocytes. D — HIT requires heparin exposure, and the long PT, long aPTT and low fibrinogen point to factor consumption, which fits DIC.

Source: Merck Manual Professional, "Disseminated Intravascular Coagulation (DIC)" — Diagnosis — https://www.merckmanuals.com/professional/hematology-and-oncology/coagulation-disorders/disseminated-intravascular-coagulation-dic; ARUP Consult, "Disseminated Intravascular Coagulation" — Distinguishing DIC from TTP and HUS — https://arupconsult.com/content/disseminated-intravascular-coagulation (both checked Nov 1, 2026)

Answer: D. Cryptococcal antigen testing on CSF or serum is rapid and more sensitive than India ink, which misses a substantial share of cases.

Why the others are wrong: A identifies Candida albicans. B dissolves keratin to reveal fungal elements in skin, hair and nails. C is used for invasive Aspergillus infection.

Source: MSD Manual Professional, "Cryptococcosis" — Diagnosis — https://www.msdmanuals.com/professional/infectious-diseases/fungi/cryptococcosis; Nalintya E, et al. review of cryptococcal antigen testing (author manuscript, CDC Stacks) — https://stacks.cdc.gov/view/cdc/39436/cdc_39436_DS1.pdf (both checked Nov 1, 2026)

Answer: C. This is the centromere pattern (anti-centromere antibodies), strongly associated with limited cutaneous systemic sclerosis.

Why the others are wrong: A — SLE is linked to homogeneous patterns (anti-dsDNA, anti-histone) and others. B — Sjögren is linked to fine speckled patterns (anti-SSA/Ro, anti-SSB/La). D — drug-induced lupus is linked to a homogeneous pattern (anti-histone).

Source: MSD Manual Professional, "Systemic Sclerosis" — Diagnosis: antibody testing — https://www.msdmanuals.com/professional/musculoskeletal-and-connective-tissue-disorders/autoimmune-rheumatic-disorders/systemic-sclerosis (checked Nov 1, 2026)

Answer: B. Group O people have naturally occurring IgG anti-A,B that crosses the placenta, so no earlier pregnancy is needed to sensitize the mother. It's usually mild, and the DAT is often weak or even negative because A and B antigens are weakly developed on fetal cells.

Why the others are wrong: A — both mother and baby are D positive. C — anti-K would appear on the mother's antibody screen. D — Lewis antibodies don't cause HDFN (and would also show on the screen).

Source: Dean L. Blood Groups and Red Cell Antigens (NCBI Bookshelf), Ch. 4 "Hemolytic disease of the newborn" — https://www.ncbi.nlm.nih.gov/books/NBK2266/; Deb et al., Transfus Apher Sci 2024;63:103864 — https://www.isbtweb.org/asset/CE3C0DE5-61B3-4D23-BDA6E7308D8D7446 (both checked Nov 1, 2026)

Answer: D. GGT rises with hepatobiliary disease but not with bone disease. An elevated ALP with an elevated GGT points to a hepatobiliary source.

Why the others are wrong: A, B and C can raise ALP, but GGT stays normal in those settings.

Source: MedlinePlus, "Gamma-glutamyl Transferase (GGT) Test" — elevated GGT with elevated ALP supports a liver/biliary source; isolated ALP elevation can reflect bone disease — https://medlineplus.gov/lab-tests/gamma-glutamyl-transferase-ggt-test/ (checked Nov 1, 2026)

Answer: A. The platelet count fell by about 64%, the timing is compatible with HIT, and there is new thrombosis. For an intermediate- or high-probability 4Ts score, ASH recommends an immunoassay; if it is positive and a functional assay is available, a functional assay such as the serotonin release assay is suggested.

Why the others are wrong: B evaluates von Willebrand factor and GPIb function. C is nonspecific — it rises with any clot. D screens platelet function and is unreliable with a low platelet count.

Source: American Society of Hematology, Diagnosis and Management of Heparin-Induced Thrombocytopenia pocket guide — 4Ts probability, immunoassay for intermediate/high probability, and functional assay after a positive immunoassay when available — https://www.hematology.org/-/media/hematology/files/education/clinicians/guidelines-quality/documents/watermarked-pocket-guides/watermark-ash_vte_hit_pocketguide.pdf (checked Nov 1, 2026)

Answer: C. These features fit Bacillus anthracis. Sentinel (clinical) laboratories perform only presumptive rule-out tests. If B. anthracis can't be ruled out, they notify the state public health laboratory, which arranges referral to a Laboratory Response Network reference laboratory for confirmation.

Why the others are wrong: A — confirmation belongs to the LRN reference laboratory; extra manipulation adds exposure risk. B — discarding a possible select agent is a serious error. D — susceptibility testing is the reference laboratory's job.

Source: Association of Public Health Laboratories, "Bacillus anthracis Rule-out Evaluation" — sentinel-laboratory rule-out/referral workflow and warning against routine susceptibility testing — https://aphl.org/docs/default-source/technical/PHPR-2020-Biothreat-Rule-Out-B-anthracis.pdf (checked Nov 1, 2026)

Answer: A. Clean the site with soap and water right away, then report it immediately. If post-exposure prophylaxis is recommended, it should start as soon as possible — ideally within hours.

Why the others are wrong: B — squeezing the wound and caustic agents such as bleach aren't recommended; there's no evidence they reduce transmission. C — delay can make prophylaxis less effective. D — source-patient testing happens through the employer's exposure process, after consent; the exposed worker shouldn't draw the patient's blood on their own.

Source: OSHA Bloodborne Pathogens Standard, 29 CFR 1910.1030(f)(3), post-exposure evaluation and follow-up — https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1030; CDC, "Best Practices for Occupational Exposure to Blood" — occupational exposures should be treated and evaluated immediately — https://www.cdc.gov/dental-infection-control/hcp/dental-ipc-faqs/occupational-exposure.html (checked Nov 1, 2026)