Biochemistry, microbiology and immunology

50 Gulf Prometric Pharmacist (DHA, DOH, MOH, SCFHS, QCHP) practice questions on Biochemistry, microbiology and immunology, part of Basic Biomedical Sciences. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   Which structure in the Gram-negative cell envelope contains lipopolysaccharide (endotoxin)?

   1. AThe thick peptidoglycan layer
   2. BThe cytoplasmic membrane
   3. CThe capsule polysaccharide
   4. DThe outer membrane

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   Answer: D · The outer membrane

   The outer leaflet of the Gram-negative outer membrane is lipopolysaccharide, whose lipid A portion triggers fever and septic shock when released. Gram-positive bacteria lack an outer membrane, and have a thick peptidoglycan layer with teichoic acids.

   • A: Gram-negative bacteria have only a thin peptidoglycan layer.
   • B: The cytoplasmic membrane is a phospholipid bilayer.
   • C: Capsules are polysaccharide, not LPS.

   Remember this

   Gram-negative outer membranes carry lipopolysaccharide, whose lipid A is the endotoxin.

2. Question 2 · Easy

   Penicillins kill bacteria by interfering with which process?

   1. ASynthesis of folic acid
   2. BBacterial protein synthesis at the 30S subunit
   3. CDNA replication by gyrase
   4. DCross-linking of peptidoglycan in the cell wall

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   Answer: D · Cross-linking of peptidoglycan in the cell wall

   Beta-lactams bind penicillin-binding proteins (transpeptidases) and stop cross-linking of peptidoglycan, so the cell wall is weak and the cell lyses. Human cells lack a cell wall, which gives selective toxicity. Activity needs growing bacteria.

   • A: Folate synthesis is blocked by sulfonamides.
   • B: 30S inhibition is a feature of aminoglycosides and tetracyclines.
   • C: DNA gyrase is inhibited by fluoroquinolones.

   Remember this

   Beta-lactams block penicillin-binding proteins and peptidoglycan cross-linking.

3. Question 3 · Medium

   Vancomycin resistance in enterococci is caused by a change in the peptidoglycan precursor from D-Ala-D-Ala to:

   1. AD-Ala-D-Lac
   2. BL-Ala-L-Lys
   3. CD-Glu-D-Ala
   4. DMuramic acid-Gly

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   Answer: A · D-Ala-D-Lac

   Vancomycin binds the terminal D-Ala-D-Ala of the precursor. In VanA and VanB strains, the terminal becomes D-Ala-D-lactate, which reduces the binding affinity about a thousand-fold. Linezolid or daptomycin may be used for VRE.

   • B: L-Ala-L-Lys is not the precursor change.
   • C: D-Glu-D-Ala is not the altered terminal.
   • D: Muramic acid-Gly is not a vancomycin target change.

   Remember this

   VanA and VanB enterococci replace D-Ala-D-Ala with D-Ala-D-Lac, losing vancomycin binding.

4. Question 4 · Medium

   A young man has a purulent urethral discharge, and Gram stain shows Gram-negative diplococci inside neutrophils. The likely organism is:

   1. ANeisseria gonorrhoeae
   2. BTreponema pallidum
   3. CEscherichia coli
   4. DChlamydia trachomatis

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   Answer: A · Neisseria gonorrhoeae

   Intracellular Gram-negative diplococci in pus cells are characteristic of N. gonorrhoeae. Chlamydia cannot be seen on Gram stain because it is an obligate intracellular organism. E. coli is a rod and T. pallidum is a spirochaete that does not Gram-stain well.

   • B: T. pallidum is a thin spirochaete seen by dark-field microscopy.
   • C: E. coli is a Gram-negative rod.
   • D: Chlamydia is not seen on Gram stain.

   Remember this

   Gram-negative intracellular diplococci in a urethral discharge indicate Neisseria gonorrhoeae.

5. Question 5 · Hard

   A woman has a urinary tract infection. Urine culture on MacConkey agar grows pink colonies of a Gram-negative rod that is indole positive. The organism is most likely:

   1. AEscherichia coli
   2. BKlebsiella pneumoniae
   3. CEnterobacter cloacae
   4. DProteus mirabilis

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   Answer: A · Escherichia coli

   Pink colonies on MacConkey agar mean lactose fermentation, and a positive indole test points to E. coli, the commonest cause of urinary infection. Klebsiella and Enterobacter also ferment lactose but are indole negative. Proteus does not ferment lactose.

   • B: Klebsiella is indole negative and forms mucoid colonies.
   • C: Enterobacter is indole negative.
   • D: Proteus is a non-lactose fermenter that swarms.

   Remember this

   E. coli ferments lactose (pink on MacConkey) and is indole positive.

6. Question 6 · Medium

   Enzyme A has a Km of 0.5 mM and enzyme B has a Km of 5 mM for the same substrate. Which statement is correct?

   1. AEnzyme A has the higher affinity for the substrate
   2. BThe enzymes have the same affinity but different Vmax
   3. CKm is the substrate concentration at which Vmax is reached
   4. DEnzyme B has the higher affinity for the substrate

   Answer and explanation in LearnFree account

7. Question 7 · Medium

   The urea breath test for Helicobacter pylori depends on the organism's production of which enzyme?

   1. AHyaluronidase
   2. BUrease
   3. CCoagulase
   4. DCatalase

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8. Question 8 · Medium

   On second exposure to an antigen, the antibody response differs from the first in being:

   1. ASlower and mainly IgM
   2. BWeaker and short lived
   3. CFaster, higher in titre and mainly IgG
   4. DIdentical to the primary response

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9. Question 9 · Medium

   The main regulatory (rate-limiting) enzyme of glycolysis is:

   1. AGlucose-6-phosphatase
   2. BFructose-1,6-bisphosphatase
   3. CPhosphofructokinase-1
   4. DPyruvate carboxylase

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10. Question 10 · Hard

    A healthcare worker has HBsAg negative, anti-HBc negative and anti-HBs 85 mIU/mL. This pattern indicates:

    1. AChronic hepatitis B infection
    2. BRecovery from natural infection
    3. CImmunity from vaccination
    4. DAcute hepatitis B infection

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11. Question 11 · Medium

    A hospital patient on antibiotics develops watery diarrhoea and Clostridioides difficile toxin is detected. Which infection-control measure is needed?

    1. AWearing a surgical mask only
    2. BAlcohol hand gel only, since it is a rapid and effective disinfectant
    3. CSoap and water hand washing, as alcohol gel does not kill spores
    4. DNo isolation because the infection is endogenous

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12. Question 12 · Medium

    Lactulose lowers blood ammonia in hepatic encephalopathy mainly because in the colon it:

    1. AAcidifies the lumen, trapping ammonia as ammonium
    2. BIs absorbed and then used by the urea cycle in the liver
    3. CInhibits glutaminase in the liver and lowers ammonia output
    4. DKills all urease-producing bacteria in the large bowel

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13. Question 13 · Medium

    Post-streptococcal glomerulonephritis is caused by which hypersensitivity mechanism?

    1. ADeposition of circulating immune complexes (type III)
    2. BIgE-mediated mast cell degranulation (type I)
    3. CAntibody against cell surface antigens (type II)
    4. DDelayed T-cell response (type IV)

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14. Question 14 · Medium

    Allopurinol reduces serum uric acid by inhibiting which enzyme?

    1. AAdenosine deaminase
    2. BHypoxanthine-guanine phosphoribosyltransferase
    3. CXanthine oxidase
    4. DUricase

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15. Question 15 · Medium

    Which of the following is an example of type II (antibody-mediated cytotoxic) hypersensitivity?

    1. ASerum sickness after equine antivenom treatment
    2. BSeasonal allergic rhinitis caused by grass pollen
    3. CContact dermatitis from a nickel watch strap
    4. DRhesus haemolytic disease of the newborn

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16. Question 16 · Hard

    Blood cultures from a patient with a central line grow Gram-positive cocci in clusters that are catalase positive, coagulase negative and novobiocin sensitive. The organism is:

    1. AEnterococcus faecalis
    2. BStaphylococcus aureus
    3. CStaphylococcus epidermidis
    4. DStaphylococcus saprophyticus

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17. Question 17 · Medium

    A man develops an itchy eczematous rash under a metal watch strap 2 days after wearing it. This is best explained by:

    1. AA type I hypersensitivity reaction mediated by IgE antibody
    2. BType II hypersensitivity mediated by cytotoxic antibody
    3. CA type IV delayed reaction mediated by sensitised T cells
    4. DType III hypersensitivity from immune complexes

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18. Question 18 · Hard

    Isoniazid is a prodrug activated by the bacterial catalase-peroxidase KatG. Its active form kills Mycobacterium tuberculosis by inhibiting:

    1. ADNA gyrase
    2. BRNA polymerase
    3. CMycolic acid synthesis
    4. DArabinogalactan synthesis

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19. Question 19 · Easy

    Staphylococcus aureus differs from other staphylococci because it is:

    1. AGram negative
    2. BCatalase negative
    3. CCoagulase positive
    4. DOxidase positive

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20. Question 20 · Easy

    Mycobacterium tuberculosis is identified on microscopy by which stain?

    1. AGiemsa stain (Romanowsky type)
    2. BIndia ink negative stain
    3. CGram stain (crystal violet and safranin)
    4. DZiehl-Neelsen (acid-fast) stain

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21. Question 21 · Medium

    Low-dose aspirin has an antiplatelet effect lasting 7 to 10 days because it:

    1. AInhibits the glycoprotein IIb/IIIa receptor on platelets
    2. BInhibits platelet phosphodiesterase and raises cAMP
    3. CReversibly blocks the platelet ADP receptor and recovers within days
    4. DIrreversibly acetylates platelet COX-1, which cannot be replaced

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22. Question 22 · Easy

    In which organ does most urea synthesis take place?

    1. AKidney
    2. BSkeletal muscle
    3. CLiver
    4. DBrain

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23. Question 23 · Hard

    A patient with very low TPMT activity starts azathioprine and develops severe neutropenia within weeks. What is the best explanation?

    1. ALess methylation of thiopurines, so active thioguanine nucleotides accumulate
    2. BIncreased xanthine oxidase activity, which raises uric acid
    3. CFaster renal clearance of the drug, so the dose is inadequate
    4. DReduced conversion to mercaptopurine in the gut wall

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24. Question 24 · Medium

    In diabetic ketoacidosis, ketone bodies accumulate because in insulin deficiency the liver:

    1. AStores more glycogen and shifts acetyl-CoA towards ketone production
    2. BReduces lipolysis in adipose tissue, so fewer fatty acids reach the liver
    3. CConverts ketone bodies to glucose faster than they are produced
    4. DMakes excess acetyl-CoA from fat oxidation, which forms ketones

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25. Question 25 · Medium

    Ciprofloxacin kills bacteria mainly by inhibiting:

    1. APeptidyl transferase
    2. BDNA gyrase
    3. CDihydrofolate reductase
    4. DCell-wall transpeptidase

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