MLT GlobeDHA exam preparation (Dubai) – page 35
700 practice MCQs for the DHA medical laboratory exam. Level: Basic to intermediate.
In a modified APC-resistance test, patient plasma is first diluted in factor V–deficient plasma. The purpose is to:
Dilution in factor V–deficient plasma corrects other factor deficiencies, making the result depend mainly on the patient's factor V, so it detects factor V Leiden with high specificity. Genotyping confirms it.
Which flow cytometry marker on the platelet surface indicates alpha-granule release (platelet activation)?
P-selectin lines the alpha-granule membrane and appears on the surface after activation and degranulation. GP Ib is present on resting platelets.
Which platelet receptor mainly mediates firm adhesion and activation by collagen after initial capture?
GP Ib–VWF provides initial tethering under high shear, and GP VI (with integrin α2β1) binds collagen directly and triggers activation. P2Y12 is an ADP receptor.
A CLL patient with WBC 300 × 10^9/L has serum potassium 7.2 mmol/L, a normal ECG and no symptoms. Heparinized plasma potassium measured promptly is 4.1 mmol/L. The best explanation is:
Fragile leukemic cells release potassium when blood clots, falsely raising serum potassium; a gently handled plasma sample shows the true value. Tumor lysis would raise potassium in both sample types.
A smoker with small cell lung cancer has proximal weakness that improves briefly after repeated exercise, dry mouth and reduced reflexes. Which antibody is most likely present?
Lambert-Eaton myasthenic syndrome, often paraneoplastic with small cell lung cancer, is caused by antibodies to presynaptic P/Q-type calcium channels. AChR antibodies define myasthenia gravis, in which weakness worsens with exercise.
Which enzymes cut DNA at recombination signal sequences to start V(D)J recombination in developing lymphocytes?
RAG1 and RAG2 introduce double-strand breaks for V(D)J joining; their loss causes T-negative, B-negative SCID. TdT only adds random N-nucleotides at the joins.
In hybridoma production of monoclonal antibodies, HAT medium kills unfused myeloma cells because they:
Aminopterin blocks de novo nucleotide synthesis, so cells need the HGPRT salvage pathway; HGPRT-deficient myeloma cells die, while hybrids get HGPRT from the B cell. Unfused B cells die because of their limited life span.
In a multicolour flow cytometry panel, the emission of FITC is partly detected in the PE channel. The correction applied for this is called:
Compensation mathematically subtracts spectral overlap of one fluorochrome into another detector. Gating selects cell populations for analysis but does not correct spillover.
Proficiency testing results disagree with the peer group, but patient samples split with another laboratory agree well. The most likely explanation is:
Processed PT materials may behave differently from patient samples on some methods (non-commutability). If patient splits agree, the method is likely correct and the matrix effect explains the PT bias.
A carryover check runs three high samples (H1–H3) then three low samples (L1–L3). H3 = 1000 U/L, L1 = 12 U/L, L3 = 10 U/L. Using (L1 − L3)/(H3 − L3) × 100, the carryover is about:
Carryover = (12 − 10)/(1000 − 10) × 100 = 2/990 × 100 ≈ 0.2%. Using L1 alone (12/1000) or dividing incorrectly gives the other values.
A neonate has meningitis with a brain abscess. The CSF grows a motile gram-negative rod that is indole positive, citrate positive, H2S negative and urease negative. The most likely organism is:
Citrobacter koseri is indole positive and H2S negative and is classically linked to neonatal meningitis with brain abscess. C. freundii is indole negative and H2S positive; E. coli is citrate negative.
A teenager has pharyngitis with a scarlet fever-like rash. Throat culture grows small beta-hemolytic colonies of a catalase-negative gram-positive rod that inhibits the hemolysis of S. aureus beta-lysin (reverse CAMP positive). The most likely organism is:
Arcanobacterium haemolyticum causes pharyngitis with rash in teenagers and young adults. It is a catalase-negative rod whose phospholipase D blocks S. aureus beta-lysin. S. pyogenes is a coccus.
Urine from an 80-year-old man grows alpha-hemolytic, catalase-negative gram-positive cocci in clusters and tetrads. The isolate is PYR negative, LAP positive and grows in 6.5% NaCl. The most likely organism is:
Aerococcus urinae is a catalase-negative coccus in clusters that is PYR negative and LAP positive; A. viridans has the reverse pattern. Enterococci are PYR positive and grow in chains; staphylococci are catalase positive.
A kidney transplant recipient has painful skin nodules. Biopsy shows acid-fast bacilli. Culture grows only on chocolate agar incubated at 30 °C, not on Löwenstein-Jensen at 37 °C. The most likely organism is:
M. haemophilum needs hemin or ferric ammonium citrate and grows best at 28–32 °C, so it is missed on routine LJ at 37 °C. M. marinum does not need hemin.
A non-pigmented mycobacterium from a surgical wound grows in 4 days. It is positive for 3-day arylsulfatase, nitrate reduction and iron uptake. The most likely species is:
M. fortuitum is a rapid grower that reduces nitrate and takes up iron. M. chelonae and M. abscessus are also arylsulfatase positive but nitrate negative.
A cryptococcal isolate turns canavanine-glycine-bromothymol blue (CGB) agar blue. This identifies the isolate as:
C. gattii resists canavanine and uses glycine, raising the pH so the indicator turns blue; C. neoformans leaves the medium yellow-green. The distinction matters for epidemiology and prognosis.
A Haemophilus influenzae isolate is beta-lactamase negative but ampicillin resistant. The most likely mechanism is:
Beta-lactamase-negative ampicillin-resistant (BLNAR) H. influenzae have mutations in the ftsI gene encoding PBP3. TEM-1 and ROB-1 are beta-lactamases and would give a positive test.
HIV genotypic drug-resistance testing usually fails when the plasma HIV-1 RNA is below:
Sequencing needs enough viral RNA to amplify the pol gene, so most assays require a viral load of roughly 500–1000 copies/mL or more. A load of 20 copies/mL is too low to sequence.
A synovial fluid shows small clumps that are not birefringent under polarised light but stain red with alizarin red S. These are most likely:
Hydroxyapatite crystals are too small to see singly and are non-birefringent; alizarin red S stains calcium-containing clumps. Urate crystals are strongly birefringent needles.
A patient on cyclophosphamide therapy also receives MESNA. The urine ketone pad is positive although there is no ketosis. The best explanation is:
Compounds with free sulfhydryl groups, such as MESNA and captopril, react with nitroprusside and give false-positive ketone results. The pad does not detect beta-hydroxybutyrate.