Answer Key – Department of Microbiology, Pre-University Exam (Batch-2024, Paper I, Section A – MCQs)
| No. | Correct Answer | Brief Rationale |
|---|---|---|
| 1 | iv. Tu Youyou discovered artemisinin | Frederick Sanger developed DNA sequencing (not PCR - that was Kary Mullis); leprosy is not "Koch's disease" (TB is); N. gonorrhoeae is readily culturable in vitro. Tu Youyou won the 2015 Nobel Prize for discovering artemisinin. |
| 2 | ii. Flow cytometry | Flow cytometry (CD4/CD8 gating) is the standard method for absolute CD4 T-cell counts. CLIA, ELISA, and ICT are antibody-detection assays for HIV, not CD4 quantification. |
| 3 | ii. Are deficient in C1, C2, C3, C4 | Deficiency of early classical-pathway complement components (C1q, C2, C4, and to a lesser extent C3) impairs immune-complex clearance and predisposes to SLE-like disease. |
| 4 | i. Type I hypersensitivity | Food allergens (including fish) act through IgE-mediated, mast-cell degranulation - Type I hypersensitivity. |
| 5 | ii. At least one month | Live vaccines not given simultaneously must be separated by a minimum of 4 weeks (1 month) to avoid interference. |
| 6 | i. By direct contact (and vertically, iii, is also true) | Ebola spreads via direct contact with infected body fluids and is documented to transmit vertically (mother-to-child). It is not transmitted by eating properly cooked food (cooking inactivates the virus), so "All of the above" (iv) is not strictly correct - the intended single-best answer is direct contact. |
| 7 | iii. Intended for symptomatic individuals with AIDS indicator disease | Under the NACO HIV Testing Strategy, Strategy/Algorithm II is diagnostic testing, split into II-A (asymptomatic) and II-B (symptomatic with an AIDS indicator disease). |
| 8 | iii. Paracoccidioidomycosis | Multiple peripheral budding yeasts around a central mother cell give the classic "pilot's/mariner's wheel" appearance on methenamine silver (GMS) stain, characteristic of Paracoccidioides brasiliensis. |
| 9 | i. Biotyping | El Tor and Classical biotypes of V. cholerae O1 are distinguished by biochemical/biotyping tests (hemolysis, Voges-Proskauer, chick cell agglutination, polymyxin B sensitivity). |
| 10 | iii. Amoebic dysentery | Amoebic dysentery classically shows dark red ("anchovy sauce") blood-stained stool, acidic pH, and scanty pus cells - contrasting with bacillary dysentery (alkaline pH, numerous pus cells). |
| 11 | iv. Better patient outcomes | Effective teamwork reduces errors and communication gaps, improving patient outcomes. |
| 12 | ii. Improper sample collection | A report of "salivary specimen" means the patient submitted saliva instead of true lower-respiratory sputum - a collection technique failure, not transport/storage/quantity. |
| 13 | Silver impregnation (the option that is not a capsule-demonstration method) | M'Fadyean stain, Quelling reaction, and negative (India ink) staining are all established capsule-demonstration methods; silver impregnation is used for spirochetes/fungal elements, not capsules. |
| 14 | d. M. tuberculosis | M. tuberculosis grows on artificial media (e.g., Lowenstein-Jensen) - it is not obligate intracellular. Rickettsia, Chlamydia, and M. leprae all require living host cells. |
| 15 | d. Schistosoma | Schistosoma cercariae actively penetrate intact skin during freshwater contact; other trematodes (Fasciola, Clonorchis, Paragonimus) infect via ingestion of metacercariae. |
| 16 | b. Low specificity | Modern ELISA-based assays are noted for high sensitivity and high specificity, large sample throughput, and cost-effectiveness - "low specificity" is not a feature. |
| 17 | c. Fibroblasts (skin) | Professional APCs are dendritic cells, macrophages, and B cells; fibroblasts lack the machinery/co-stimulatory molecules to function as professional APCs. |
| 18 | b. Subclavian vein insertion | Catheter infection risk is femoral > internal jugular > subclavian; subclavian access carries the lowest CRBSI risk, making it the exception among listed risk factors. |
| 19 | c. Tick | Kyasanur Forest Disease (monkey fever) is transmitted by the bite of Haemaphysalis spinigera ticks. |
| 20 | c. Loa loa | Loa loa microfilariae show diurnal periodicity, peaking in peripheral blood during daytime; Wuchereria bancrofti and Brugia species show nocturnal periodicity. |
- Q6 (Ebola) and Q13 (capsule stain) had slightly ambiguous option formatting in the photo (possible OCR/line-wrap artifacts) - I've given the most defensible single answer with the reasoning spelled out so you can cross-check against your own copy if needed.
- These answers were cross-verified against standard references (Jawetz, Melnick & Adelberg's Medical Microbiology; Sabiston/Fischer's Surgery for CRBSI; Andrews'/Henry's for SLE complement deficiency) via the medical textbook library.
You are answering Part B (Essay + Short Notes) of a MBBS/paramedical Department of Microbiology pre-university exam paper. Use the query_library_db tool (medical textbook library, FTS5 search over paragraphs_fts and headings_fts) to verify facts, then write complete, exam-ready answers. Questions to answer: ESSAY Q1 (20 marks - 4x5): A 68-year-old man is hospitalized with a 3-day history of high fever, productive cough with rust-coloured sputum, and worsening shortness of breath. Past history: severe COPD requiring home oxygen. Physical exam: temperature 102°F (38.9°C), BP 108/62 mmHg, HR 102/min, RR 24/min. Auscultation: decreased breath sounds and crackles over the right lower lung field. Patient started empirically on IV cefepime and azithromycin. 1. Name the most likely causative organism (justify using the rust-coloured sputum clue and clinical picture - classic teaching link). 2. How do you test its antibiotic susceptibility? (methods for this organism specifically) 3. Briefly discuss the different methods of Antimicrobial Susceptibility Testing (AST) in general (disk diffusion/Kirby-Bauer, broth dilution/MIC, E-test, automated systems, molecular methods). 4. Write briefly about the antibiotic resistance mechanisms relevant to this organism/scenario (e.g., altered PBPs / penicillin-binding protein changes in Streptococcus pneumoniae, macrolide resistance mechanisms - efflux pump mef, ribosomal methylation erm). SHORT NOTES Q2 (10 marks - write a short paragraph, 4-6 sentences each, for each): 1. PCR and its applications - discuss different types of PCR (conventional, real-time/qPCR, RT-PCR, multiplex, nested, RAPD, digital PCR) and its advantages and disadvantages. 2. Laboratory diagnosis of Malaria (peripheral blood smear thick/thin, QBC, rapid diagnostic tests/antigen detection, PCR, serology). 3. Scrub typhus (causative agent Orientia tsutsugamushi, vector, clinical features - eschar, Weil-Felix test, IgM ELISA, PCR). 4. Laboratory diagnosis of Typhoid fever (blood culture gold standard, Widal test, stool/urine culture, PCR, Typhidot). VERY SHORT NOTES Q3 (10 marks - 2-3 sentences each): 1. Enterotest (string test for Giardia lamblia duodenal sampling) 2. Plasmid (definition, types, significance in resistance transfer) 3. Viruses causing Hepatitis (Hep A, B, C, D, E - brief classification, transmission route each) 4. Complications of Dengue fever (DHF, DSS, hemorrhage, organ involvement) 5. Prozone phenomenon (definition in serological agglutination tests, cause, how to overcome by dilution) For each topic, run 2-3 targeted query_library_db searches against paragraphs_fts / headings_fts (e.g. MATCH 'scrub typhus' or MATCH '"Weil-Felix"') to pull authoritative textbook facts and cite the book title. Keep citations light (Book Title, general reference) since page numbers aren't always available. Return your FINAL answer as a complete, well-organized write-up covering ALL of the above questions with clear headings (Essay Q1 parts 1-4, Short Notes Q2 items 1-4, Very Short Notes Q3 items 1-5), ready to hand to a student as model answers. Do not include your search process, just the final polished answer text.
You are answering Part C (Essay + Short Notes) of a MBBS/paramedical Department of Microbiology pre-university exam paper. Use the query_library_db tool (medical textbook library, FTS5 search over paragraphs_fts and headings_fts) to verify facts, then write complete, exam-ready answers. Questions to answer: ESSAY Q1 (20 marks - 4x5): A 28-year-old man comes to the emergency department with nausea and repetitive vomiting starting 2 hours after eating a late-night meal of leftover fried rice at a Chinese restaurant. He had no diarrhoea, fever, or bloody stools. His temperature is 37°C (98.6°F). Blood pressure is 115/75 mmHg and pulse rate is 82/min. Abdominal examination shows mild diffuse tenderness without rebound or guarding. 1. Name the organism responsible for this clinical condition (justify - classic association of reheated/leftover fried rice with rapid-onset vomiting, incubation 1-6 hours = emetic toxin, cereulide, of Bacillus cereus). 2. Discuss different types of food poisoning (bacterial food intoxication vs food-borne infection) with examples and pathogenesis - cover Staphylococcus aureus (enterotoxin, preformed toxin, short incubation), Bacillus cereus (emetic and diarrheal syndromes), Clostridium perfringens, Clostridium botulinum, Salmonella, and Vibrio parahaemolyticus as relevant examples. 3. Describe the laboratory diagnosis of food poisoning (stool/vomitus/food sample culture, toxin detection assays, ELISA, PCR, epidemiological investigation). SHORT NOTES Q2 (10 marks - short paragraph each, 4-6 sentences): 1. Needle stick injury (risk, pathogens transmitted - HBV/HCV/HIV, immediate management/post-exposure prophylaxis, reporting) 2. Viral haemorrhagic fevers (examples - Ebola, Dengue, Crimean-Congo, Lassa; general features, transmission, management principles) 3. Discuss different types of hypersensitivity reactions with examples (Type I-IV, Coombs and Gell classification, examples of each) 4. ELISA - types (direct, indirect, sandwich, competitive), principles, uses, advantages and disadvantages VERY SHORT NOTES Q3 (10 marks - 2-3 sentences each): 1. Name different staining techniques used in Mycology (KOH mount, Lactophenol cotton blue/LPCB, India ink, Gomori methenamine silver/GMS, PAS, Mucicarmine, Calcofluor white) 2. Opsonization (definition, role of complement C3b and antibody in enhancing phagocytosis) 3. Name hereditary immunodeficiency disorders of phagocytes (Chronic Granulomatous Disease, Leukocyte Adhesion Deficiency, Chediak-Higashi syndrome, Job/Hyper-IgE syndrome, Myeloperoxidase deficiency) 4. Plasma sterilization (hydrogen peroxide gas plasma sterilization principle, uses for heat-sensitive equipment) 5. Slit sampler method (air sampling technique for microbiological air quality monitoring, principle of slit sampler device) For each topic, run 2-3 targeted query_library_db searches against paragraphs_fts / headings_fts (e.g. MATCH '"Bacillus cereus" AND emetic' or MATCH '"needle stick"') to pull authoritative textbook facts and cite the book title. Keep citations light (Book Title, general reference) since page numbers aren't always available. Return your FINAL answer as a complete, well-organized write-up covering ALL of the above questions with clear headings (Essay Q1 parts 1-3, Short Notes Q2 items 1-4, Very Short Notes Q3 items 1-5), ready to hand to a student as model answers. Do not include your search process, just the final polished answer text.
"Bacillus cereus" AND foodborne
"needlestick injury" AND "post-exposure prophylaxis"
"Streptococcus pneumoniae" AND "antimicrobial resistance"
"malaria" AND "rapid diagnostic test"
PART B
Essay Q1 (20 Marks) – 68-year-old man, COPD, rust-coloured sputum, RLL crackles, started on IV cefepime + azithromycin
- Oxacillin disk screen (1 µg) on 5% sheep-blood Mueller-Hinton agar, incubated in 5% CO₂: zone ≥20 mm = susceptible; <20 mm must be confirmed by MIC (cannot call resistance from the disk alone).
- MIC by broth microdilution or E-test - mandatory for penicillin/cephalosporins since CLSI breakpoints differ for meningitis vs non-meningitis isolates.
- Automated systems (Vitek-2, Phoenix) for routine reporting of penicillin, ceftriaxone, erythromycin, levofloxacin MICs.
- Molecular detection of mef, erm, mosaic pbp genes by PCR in reference labs.
| Method | Principle | Note |
|---|---|---|
| Disk diffusion (Kirby-Bauer) | Zone of inhibition around drug-impregnated disk, compared to CLSI/EUCAST breakpoints | Qualitative (S/I/R) |
| Broth/agar dilution | Serial 2-fold dilutions -> lowest conc. inhibiting growth = MIC | Gold standard, quantitative |
| E-test (gradient diffusion) | Strip with continuous drug gradient, elliptical zone | Quantitative MIC, good for fastidious organisms |
| Automated systems | Micro-well panels, photometric growth detection | Rapid, high-throughput |
| Molecular methods | PCR/sequencing for resistance genes (mecA, vanA, blaKPC) | Detects genotype, not always phenotype |
| β-lactamase tests (nitrocefin disk) | Direct enzymatic assay | Rapid presumptive screen |
- β-lactam resistance in pneumococcus is NOT via β-lactamase, but via altered/mosaic Penicillin-Binding Proteins (PBP1a, 2b, 2x) acquired through transformation from related streptococci - reduced β-lactam affinity, graded (not all-or-none) resistance.
- Macrolide (azithromycin) resistance occurs via two mechanisms: (i) mef(A/E)-mediated efflux pump* - low/moderate resistance (M-phenotype); (ii) erm(B)-mediated 23S rRNA methylation* - high-level resistance with cross-resistance to clindamycin/streptogramin B (MLSB phenotype).
Short Notes Q2 (10 Marks)
Very Short Notes Q3 (10 Marks)
- Enterotest (string test): Gelatin capsule with coiled nylon string swallowed; string end taped outside mouth, withdrawn after a few hours; duodenal mucus examined microscopically for Giardia lamblia trophozoites - alternative to duodenal aspiration.
- Plasmid: Extrachromosomal, circular, self-replicating double-stranded DNA in bacteria. Types: F (fertility), R (resistance), Col, virulence plasmids. R-plasmids spread antibiotic resistance genes between bacteria via conjugation.
- Viruses causing Hepatitis: HAV (RNA picornavirus, faeco-oral, self-limiting), HBV (DNA hepadnavirus, parenteral/sexual/perinatal, can be chronic, → HCC), HCV (RNA flavivirus, parenteral, high chronicity), HDV (defective RNA virus, needs HBV co-infection), HEV (RNA hepevirus, faeco-oral, severe in pregnancy).
- Complications of Dengue: Dengue Haemorrhagic Fever (plasma leakage, thrombocytopenia, bleeding), Dengue Shock Syndrome (circulatory collapse), hepatic involvement, myocarditis, encephalopathy, acute kidney injury, multi-organ dysfunction.
- Prozone phenomenon: False-negative/weak agglutination at low serum dilution (antibody excess) in tests like VDRL/Widal, preventing lattice formation; overcome by serially diluting the serum to reach the zone of equivalence.
PART C
Essay Q1 (20 Marks) – 28-year-old man, vomiting 2 hours after leftover fried rice, no fever/diarrhoea
- Staph aureus - heat-stable enterotoxins (superantigens acting on vagal afferents/CNS); cream pastries, ham, dairy; 1-6h.
- B. cereus (emetic) - cereulide; fried rice; 1-6h.
- C. botulinum - botulinum neurotoxin blocking ACh release at NMJ; improperly canned food, honey; 12-36h; descending flaccid paralysis, no GI/fever.
- Salmonella (non-typhoidal) - mucosal invasion + enterotoxin; poultry/eggs; 12-48h; fever, bloody diarrhoea.
- C. perfringens (Type A) - spores survive cooking, germinate in reheated meat, enterotoxin released during sporulation in gut; 8-16h; watery diarrhoea, no fever.
- B. cereus (diarrhoeal) - heat-labile enterotoxin formed in vivo; meat/sauces; 8-16h.
- V. parahaemolyticus - invasion + thermostable direct haemolysin; raw seafood; 4-30h.
- Epidemiological/food history (incubation period, attack rate, common exposure).
- Sample collection: stool, vomitus, leftover food.
- Gram stain/microscopy of food or stool.
- Culture on selective media - MacConkey/XLD/SS (Salmonella), Mannitol Salt Agar (S. aureus), PEMBA (B. cereus), TCBS (Vibrio), anaerobic blood agar (C. perfringens/botulinum). Quantitative food culture (≥10⁵ organisms/g) supports B. cereus/C. perfringens diagnosis.
- Toxin detection: RPLA/ELISA for S. aureus enterotoxin and B. cereus toxins (BCET-RPLA), mouse bioassay/neutralization for botulinum toxin, ELISA for C. perfringens enterotoxin.
- PCR for toxin genes (ces, cpe, bont).
- Rising antibody titre (Salmonella, less useful acutely).
Short Notes Q2 (10 Marks)
- Type I (IgE-mediated, immediate): anaphylaxis, asthma, allergic rhinitis, food allergy.
- Type II (cytotoxic, antibody-mediated): haemolytic disease of newborn, autoimmune haemolytic anaemia, Goodpasture syndrome, transfusion reactions.
- Type III (immune complex-mediated): serum sickness, Arthus reaction, SLE, post-streptococcal GN, ENL reaction in leprosy.
- Type IV (delayed, T-cell mediated): Mantoux/tuberculin reaction, contact dermatitis, tuberculoid leprosy, chronic graft rejection.
Very Short Notes Q3 (10 Marks)
- Mycology staining techniques: KOH mount, Lactophenol Cotton Blue (LPCB), India ink (Cryptococcus capsule), Gomori Methenamine Silver (GMS), Periodic Acid-Schiff (PAS), Mucicarmine (Cryptococcus capsule in tissue), Calcofluor white (fluorescent chitin stain).
- Opsonization: Coating of pathogens with opsonins (IgG, complement C3b/C3bi) that bind Fc/complement receptors on phagocytes, enhancing recognition and phagocytosis.
- Hereditary immunodeficiency of phagocytes: Chronic Granulomatous Disease (NADPH oxidase defect), Leukocyte Adhesion Deficiency (CD18 integrin defect), Chediak-Higashi syndrome (LYST gene, giant granules), Job/Hyper-IgE syndrome (STAT3 mutation), Myeloperoxidase deficiency.
- Plasma sterilization: Hydrogen peroxide gas plasma generates free radicals via electric field that destroy microorganisms including spores at low temperature (~45-50°C), leaving no toxic residue - used for heat/moisture-sensitive equipment like endoscopes.
- Slit sampler method: Volumetric air-sampling device where air is drawn through a narrow slit at fixed flow rate onto a rotating agar plate, allowing continuous, time-related quantification of viable airborne microorganisms (used in OT/ICU/clean-room air quality monitoring).