Q21: A patient’s serum reacts with all screening cells at the antihuman globulin phase, and the autocontrol is positive. What is the most likely explanation?
- [A] Alloantibody to a high-frequency antigen
- [B] Cold autoantibody
- [C] Warm autoantibody ✔ (Correct Answer)
- [D] ABO incompatibility
💡 Explanation:
Panreactivity at AHG with a positive autocontrol suggests a warm autoantibody, especially when reactivity occurs at 37°C/AHG.
Why other options are wrong:
• A — An antibody to a high-frequency antigen often has a negative autocontrol.
• B — Cold autoantibodies commonly react at lower temperatures.
• D — ABO incompatibility does not cause this antibody-screen pattern.
📌 Exam Pearl: Adsorption studies may be required to identify underlying alloantibodies in patients with warm autoantibodies.
Q22: Which urinalysis finding is most characteristic of nephrotic syndrome?
- [A] Large numbers of dysmorphic RBCs
- [B] Oval fat bodies ✔ (Correct Answer)
- [C] WBC casts
- [D] Hexagonal crystals
💡 Explanation:
Oval fat bodies are renal tubular epithelial cells containing lipid droplets and are typical of nephrotic syndrome.
Why other options are wrong:
• A — Dysmorphic RBCs suggest glomerular bleeding.
• C — WBC casts suggest pyelonephritis or interstitial nephritis.
• D — Hexagonal crystals suggest cystinuria.
📌 Exam Pearl: Fatty casts and oval fat bodies create a “Maltese cross” under polarized light.
Q23: A serum glucose result is 3.1 mmol/L in a patient whose sample remained unseparated at room temperature for 6 hours. What is the most likely pre-analytical cause?
- [A] Glycolysis by blood cells ✔ (Correct Answer)
- [B] Lipemia interference
- [C] Increased bilirubin production
- [D] Contamination with EDTA
💡 Explanation:
Unseparated blood cells metabolize glucose through glycolysis, causing falsely low glucose concentration.
Why other options are wrong:
• B — Lipemia affects photometric assays but does not explain delayed glucose loss.
• C — Bilirubin does not cause this pattern.
• D — EDTA contamination more often causes high potassium and low calcium.
📌 Exam Pearl: Separate serum/plasma promptly or use glycolysis-inhibiting collection methods when delay is expected.
Q24: Which RBC index is calculated as: $\text{MCV} = \frac{\text{Hematocrit} \times 10}{\text{RBC count}}$?
- [A] MCH
- [B] MCHC
- [C] MCV ✔ (Correct Answer)
- [D] RDW
💡 Explanation:
The stated formula calculates mean corpuscular volume (MCV).
Why other options are wrong:
• A — MCH is hemoglobin divided by RBC count.
• B — MCHC is hemoglobin divided by hematocrit.
• D — RDW measures RBC size variability.
📌 Exam Pearl: MCV is expressed in femtoliters (fL) and classifies anemia as microcytic, normocytic, or macrocytic.
Q25: A sputum Gram stain shows many neutrophils, few squamous epithelial cells, and numerous lancet-shaped Gram-positive diplococci. Which organism is most likely?
- [A] Streptococcus pneumoniae ✔ (Correct Answer)
- [B] Staphylococcus epidermidis
- [C] Neisseria meningitidis
- [D] Haemophilus influenzae
💡 Explanation:
Lancet-shaped Gram-positive diplococci in a high-quality sputum specimen are typical of Streptococcus pneumoniae.
Why other options are wrong:
• B — S. epidermidis forms clusters and is often skin flora.
• C — N. meningitidis is Gram-negative diplococci.
• D — H. influenzae is a small pleomorphic Gram-negative coccobacillus.
📌 Exam Pearl: Few squamous epithelial cells indicate a lower-respiratory specimen with minimal oral contamination.
Q26: A patient has a positive hepatitis B surface antigen (HBsAg) and positive hepatitis B core IgM antibody (anti-HBc IgM). Which interpretation is most appropriate?
- [A] Immunity due to vaccination
- [B] Acute hepatitis B infection ✔ (Correct Answer)
- [C] Resolved hepatitis B infection
- [D] Chronic infection with immunity
💡 Explanation:
HBsAg with anti-HBc IgM indicates acute hepatitis B infection.
Why other options are wrong:
• A — Vaccination causes anti-HBs alone.
• C — Resolved infection usually has anti-HBs and total anti-HBc but lacks HBsAg.
• D — Chronic infection typically lacks anti-HBc IgM.
📌 Exam Pearl: Anti-HBc IgM is a primary marker of recent/acute HBV infection.
Q27: Which stain is routinely used to demonstrate acid-fast bacilli in a sputum specimen?
- [A] Gram stain
- [B] India ink stain
- [C] Ziehl-Neelsen stain ✔ (Correct Answer)
- [D] Wright stain
💡 Explanation:
Ziehl-Neelsen stain detects acid-fast bacilli due to mycolic acids present in the cell wall.
Why other options are wrong:
• A — Gram stain poorly stains mycobacteria.
• B — India ink demonstrates Cryptococcus capsule.
• D — Wright stain is used for blood cells and blood parasites.
📌 Exam Pearl: Acid-fast smear positivity does not distinguish M. tuberculosis complex from nontuberculous mycobacteria.
Q28: During a blood transfusion, a patient develops fever, flank pain, hypotension, and hemoglobinuria. What is the first laboratory/clinical action?
- [A] Continue the transfusion at a slower rate
- [B] Stop the transfusion and initiate transfusion-reaction investigation ✔ (Correct Answer)
- [C] Administer an antihistamine and resume transfusion
- [D] Issue another unit from the same donor
💡 Explanation:
Symptoms of acute hemolytic reaction require immediate transfusion cessation and formal investigation.
Why other options are wrong:
• A — Slowing the transfusion is unsafe.
• C — Antihistamines do not treat suspected hemolysis.
• D — Another unit from the same donor is inappropriate before investigation.
📌 Exam Pearl: Maintain IV access with appropriate normal saline and notify the clinical team and blood bank immediately.
Q29: Which analyte is most useful for assessing short-term glycemic control over approximately 2–3 weeks?
- [A] Hemoglobin A1c
- [B] Fructosamine ✔ (Correct Answer)
- [C] C-peptide
- [D] Fasting insulin
💡 Explanation:
Fructosamine reflects glycation of serum proteins and approximates glycemic control over the previous 2–3 weeks.
Why other options are wrong:
• A — HbA1c reflects approximately 2–3 months.
• C — C-peptide reflects endogenous insulin secretion.
• D — Fasting insulin does not assess average glycemia.
📌 Exam Pearl: Fructosamine is useful when HbA1c is unreliable because of altered RBC survival (e.g., hemolytic anemia).
Q30: A tissue section shows weak nuclear staining with hematoxylin. Which corrective action is most appropriate?
- [A] Increase bluing time
- [B] Shorten dehydration time
- [C] Increase hematoxylin staining time ✔ (Correct Answer)
- [D] Replace xylene with alcohol
💡 Explanation:
Weak hematoxylin staining is corrected by increasing hematoxylin staining time or ensuring reagent freshness and concentration.
Why other options are wrong:
• A — Bluing changes the color shift, not initial weak nuclear uptake.
• B — Dehydration occurs later in the staining process.
• D — Xylene is unrelated to nuclear staining intensity.
📌 Exam Pearl: Hematoxylin stains nucleic acids (nuclei); eosin stains cytoplasm and extracellular matrix.
Q31: A peripheral smear shows macro-ovalocytes and hypersegmented neutrophils. Which deficiency is most likely?
- [A] Iron deficiency
- [B] Vitamin B12 or folate deficiency ✔ (Correct Answer)
- [C] Glucose-6-phosphate dehydrogenase deficiency
- [D] Thalassemia trait
💡 Explanation:
Macro-ovalocytes and hypersegmented neutrophils are classic hallmarks of megaloblastic anemia caused by Vitamin B12 or folate deficiency.
Why other options are wrong:
• A — Iron deficiency produces microcytic hypochromic RBCs.
• C — G6PD deficiency produces bite cells and Heinz bodies.
• D — Thalassemia trait produces target cells and microcytosis.
📌 Exam Pearl: Evaluate vitamin B12 and folate levels promptly because B12 deficiency can cause irreversible neurological complications.
Q32: Which organism is most commonly associated with pseudomembranous colitis after antibiotic therapy?
- [A] Clostridioides difficile ✔ (Correct Answer)
- [B] Campylobacter jejuni
- [C] Salmonella enterica
- [D] Shigella sonnei
💡 Explanation:
C. difficile produces enterotoxin (Toxin A) and cytotoxin (Toxin B) that cause antibiotic-associated pseudomembranous colitis.
Why other options are wrong:
• B — Campylobacter causes inflammatory diarrhea without classic pseudomembranes.
• C — Salmonella causes acute gastroenteritis.
• D — Shigella causes bacillary dysentery.
📌 Exam Pearl: Test unformed (liquid/soft) stool samples from symptomatic patients for toxin detection.
Q33: A laboratory receives a specimen labeled with two different patient identifiers that do not match the request form. What is the most appropriate action?
- [A] Correct the label using the request form
- [B] Process the specimen if the test is urgent
- [C] Reject the specimen and request recollection ✔ (Correct Answer)
- [D] Ask the ward clerk which identifier is correct
💡 Explanation:
Mismatched identifiers create an unacceptable risk of wrong-patient error. Rejection and recollection are mandatory.
Why other options are wrong:
• A — Staff must never relabel primary specimens retrospectively.
• B — Clinical urgency does not override patient safety identification protocols.
• D — Verbal confirmations are insufficient for resolving mismatched labeling errors.
📌 Exam Pearl: Proper patient identification is a primary quality control barrier in pre-analytical testing.
Q34: A newborn has a positive DAT and elevated unconjugated bilirubin. The mother is group O, and the infant is group A. Which condition is most likely?
- [A] ABO hemolytic disease of the fetus and newborn ✔ (Correct Answer)
- [B] Hemophilia A
- [C] Hereditary spherocytosis
- [D] Vitamin K deficiency
💡 Explanation:
Group O mothers naturally possess IgG anti-A,B antibodies that cross the placenta and target group A or B fetal RBCs, leading to ABO HDFN.
Why other options are wrong:
• B — Hemophilia A causes bleeding disorders, not positive DAT or immune hemolysis.
• C — Hereditary spherocytosis is a membrane defect, independent of maternal blood group.
• D — Vitamin K deficiency causes hemorrhagic disease of the newborn.
📌 Exam Pearl: ABO HDFN is often milder than Rh-mediated disease but frequently presents in first pregnancies.
Q35: Which crystal is most often found in acidic urine?
- [A] Triple phosphate
- [B] Amorphous phosphate
- [C] Calcium phosphate
- [D] Uric acid ✔ (Correct Answer)
💡 Explanation:
Uric acid crystals precipitate predominantly in acidic urine ($\text{pH} < 5.7$).
Why other options are wrong:
• A — Triple phosphate crystals form in alkaline urine.
• B — Amorphous phosphates precipitate in alkaline urine.
• C — Calcium phosphate crystals are found in neutral to alkaline urine.
📌 Exam Pearl: Uric acid crystals show pleomorphism (yellow-brown rhombic, rosette, or diamond shapes) under light microscopy.
Q36: A patient has sodium 140 mmol/L, chloride 104 mmol/L, and bicarbonate 12 mmol/L. What is the calculated anion gap?
- [A] 12 mmol/L
- [B] 20 mmol/L
- [C] 24 mmol/L ✔ (Correct Calculated Result)
- [D] 36 mmol/L
💡 Explanation:
The formula for Anion Gap is: $\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)$.
$\text{AG} = 140 - (104 + 12) = 140 - 116 = 24\text{ mmol/L}$.
📌 Exam Pearl: An elevated anion gap ($> 12\text{ mmol/L}$) indicates high anion gap metabolic acidosis (e.g., ketoacidosis, lactic acidosis).
Q37: Which coagulation factor deficiency usually prolongs the aPTT but not the PT?
- [A] Factor VII deficiency
- [B] Factor VIII deficiency ✔ (Correct Answer)
- [C] Factor V deficiency
- [D] Factor X deficiency
💡 Explanation:
Factor VIII belongs exclusively to the intrinsic pathway; its deficiency prolongs aPTT while PT remains normal.
Why other options are wrong:
• A — Factor VII is part of the extrinsic pathway and prolongs PT only.
• C — Factor V is part of the common pathway and prolongs both PT and aPTT.
• D — Factor X is part of the common pathway and prolongs both PT and aPTT.
📌 Exam Pearl: Hemophilia A is Factor VIII deficiency, whereas Hemophilia B is Factor IX deficiency.
Q38: A urine culture from a properly collected midstream sample grows more than 100,000 CFU/mL of a single lactose-fermenting Gram-negative rod. Which next step is most appropriate?
- [A] Report mixed urogenital flora
- [B] Perform identification and antimicrobial susceptibility testing ✔ (Correct Answer)
- [C] Reject the culture as contaminated
- [D] Incubate anaerobically for 48 hours
💡 Explanation:
Significant pure growth ($\ge 10^5\text{ CFU/mL}$) from a clean-catch midstream urine indicates a true UTI requiring species ID and AST.
Why other options are wrong:
• A — Mixed flora applies when multiple organisms grow in low counts.
• C — Pure growth of $\ge 10^5\text{ CFU/mL}$ represents pathogen growth, not contamination.
• D — Routine urine pathogens are aerobic or facultative anaerobes.
📌 Exam Pearl: Escherichia coli is the predominant lactose-fermenting Gram-negative rod in community-acquired UTIs.
Q39: Which histochemical stain is used to demonstrate iron in bone marrow aspirates?
- [A] Congo red
- [B] Prussian blue ✔ (Correct Answer)
- [C] Periodic acid-Schiff
- [D] Ziehl-Neelsen
💡 Explanation:
Prussian blue (Perls' stain) reacts with ferric iron to form insoluble ferric ferrocyanide, demonstrating storage iron.
Why other options are wrong:
• A — Congo red is specific for amyloid deposits.
• C — PAS stains glycogen, mucins, and fungal walls.
• D — Ziehl-Neelsen demonstrates acid-fast bacteria.
📌 Exam Pearl: Absence of stainable iron in bone marrow confirms iron deficiency anemia.
Q40: A blood bank technologist detects a clinically significant anti-K antibody in a recipient. Which RBC units should be selected?
- [A] K-positive, AHG crossmatch compatible
- [B] K-negative, crossmatch compatible ✔ (Correct Answer)
- [C] Group O RBCs regardless of K antigen status
- [D] Washed RBCs regardless of K antigen status
💡 Explanation:
Anti-K is a IgG, clinically significant antibody that causes severe hemolytic transfusion reactions. K-negative crossmatch-compatible units are required.
Why other options are wrong:
• A — Transfusing K-positive units causes acute/delayed hemolysis.
• C — ABO group selection does not protect against Kell antigen reactions.
• D — Washing removes donor plasma proteins, not RBC membrane antigens.
📌 Exam Pearl: Antigen-negative RBC selection is mandatory for clinically significant red cell alloantibodies.
