Syndromic Diagnosis (CNS, Bloodstream, Respiratory, GI Infection Work-up) MCQs

Microbiology · 21 free questions with answers & explanations.

1. A 60-year-old diabetic presents with fever (38.9°C), tachycardia, hypotension, and altered sensorium. Blood cultures are drawn (2 sets, aerobic + anaerobic), and empirical broad-spectrum antibiotics are started. Regarding the timing of blood cultures in sepsis, which statement is CORRECT?
2. A 50-year-old diabetic develops meningitis. CSF shows: opening pressure 280 mmH₂O, turbid fluid, protein 220 mg/dL, glucose 18 mg/dL (blood glucose 90 mg/dL), 1800 cells/mm³ predominantly neutrophils. Gram stain shows short gram-positive rods. What is the most likely organism and the treatment?
3. Community-acquired pneumonia in a 70-year-old resident of a nursing home shows lobar consolidation and blood cultures grow gram-negative rods with a mucoid 'string test' positive (mucoid string >5 mm). Sputum Gram stain shows encapsulated gram-negative rods with bipolar staining. The organism and its clinical significance are:
4. A 60-year-old diabetic presents with bacterial meningitis. CSF analysis: turbid, WBC 2800 cells/µL (95% neutrophils), protein 280 mg/dL, glucose 22 mg/dL (simultaneous blood glucose 120 mg/dL). Gram stain shows gram-positive lancet-shaped diplococci. An automated blood culture system flags positive at 12 hours. Which combination correctly describes the MOST important virulence factor of this organism and the optimal antibiotic choice for penicillin-susceptible strains?
5. A 70-year-old man with a urinary catheter for 5 days develops fever and rigors. Blood cultures ×2 grow Pseudomonas aeruginosa (non-mucoid, oxidase positive, grape-like odour, produces pyocyanin). Disc diffusion shows resistance to piperacillin, imipenem, and gentamicin; sensitive only to colistin (polymyxin E) and ceftazidime-avibactam. What resistance mechanism would explain combined imipenem and piperacillin resistance in P. aeruginosa?
6. A 6-year-old unvaccinated child presents with acute bacterial meningitis. CSF analysis: turbid, WBC 4500 cells/µL (95% neutrophils), glucose 10 mg/dL (blood glucose 90 mg/dL), protein 380 mg/dL. Gram stain shows gram-negative diplococci. What is the CSF:serum glucose ratio and what does it indicate?
7. A 65-year-old diabetic patient presents with fever, cough, and purulent sputum. His sputum Gram stain shows Gram-positive diplococci. The sputum culture grows alpha-haemolytic colonies on blood agar, bile-soluble, optochin-sensitive. The most likely pathogen and first-line outpatient treatment for mild CAP in a non-beta-lactam-allergic patient are:
8. CSF analysis from a 45-year-old immunocompetent patient with bacterial meningitis shows: WBC 2800 cells/µL (90% PMN), protein 350 mg/dL, glucose 20 mg/dL (simultaneous blood glucose 100 mg/dL — CSF:blood glucose ratio 0.2). Gram stain shows Gram-positive diplococci. Which test on CSF most rapidly confirms the specific aetiology without waiting for culture?
9. A 65-year-old immunocompetent patient presents with acute bacterial meningitis. CSF shows: glucose 28 mg/dL (serum glucose 90), protein 380 mg/dL, cells 4200 (90% polymorphs). Gram stain shows Gram-positive diplococci. What is the correct empirical antibiotic regimen AND the rationale for adding dexamethasone?
10. A 65-year-old diabetic patient presents with bacterial meningitis. CSF: turbid, opening pressure 320 mmH2O, protein 280 mg/dL, glucose 18 mg/dL (blood glucose 95 mg/dL), leucocytes 2400/µL (90% neutrophils). Gram stain shows Gram-negative diplococci. PCR for meningococcal 16S rRNA is pending. Which empirical treatment should be initiated immediately while awaiting culture sensitivity results?
11. A 19-year-old college student presents with high fever, headache, photophobia, and neck stiffness. CSF analysis: turbid, WBC 2,400/µL (95% neutrophils), protein 280 mg/dL, glucose 22 mg/dL (blood glucose 94 mg/dL). Gram stain shows gram-negative diplococci intracellularly. The most appropriate empiric antibiotic therapy is:
12. Blood cultures are drawn from a febrile patient before antibiotic initiation. The optimal timing and volume for adult blood cultures to maximise yield is:
13. Syndromic respiratory panel (BioFire FilmArray) detects multiple pathogens simultaneously from nasopharyngeal swab. Its major limitation compared to conventional culture is:
14. A 45-year-old immunocompetent adult with no prior antibiotic therapy presents with acute bacterial meningitis (fever, neck stiffness, photophobia). CSF shows glucose 18 mg/dL (serum 90 mg/dL), protein 420 mg/dL, TLC 3200 cells/µL with 92% neutrophils. Gram stain is negative. Which empirical antibiotic combination is most appropriate pending culture results?
15. A patient with community-acquired pneumonia has a urinary antigen test (UAT) that is positive for Legionella. Which serogroup of Legionella pneumophila does the widely used Binax NOW urinary antigen test detect, and what is its sensitivity limitation?
16. A blood culture set consisting of one aerobic and one anaerobic bottle is collected from a febrile patient. The aerobic bottle turns positive at 14 hours with gram-positive cocci in clusters. The lab reports it as coagulase-negative Staphylococcus (CoNS). Under which circumstances should a single positive blood culture for CoNS be considered clinically significant?
17. A 5-year-old child returns from a week-long rural camp and develops acute watery diarrhoea with mucus (no blood) after 3 days. Microscopy of a fresh stool reveals oval cysts (8–12 µm), each containing 4 nuclei with eccentric karyosome and fibril-like axonemes (median bodies). What is the most likely pathogen and treatment of choice?
18. CSF analysis in a 25-year-old with acute bacterial meningitis typically shows which combination?
19. A 5-year-old child presents with bloody diarrhoea, fever, and on day 5 develops oliguria, thrombocytopenia, and haemolytic anaemia (peripheral smear: schistocytes). The causative organism and mechanism of renal injury is BEST described by:
20. When evaluating suspected community-acquired pneumonia (CAP), which combination of investigations provides the highest diagnostic yield for both typical and atypical pathogens?
21. A patient presents with 2 weeks of diarrhoea after hiking and drinking stream water. Stool microscopy for ova and parasites is negative on 3 samples. NAAT for gastrointestinal pathogens is positive for Giardia lamblia. Appropriate treatment is: