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LONG ESSAYS (2 × 10 = 20 marks)

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Q1. (Suspected pathogen: Influenza virus - ARDS, antigenic Shift & Drift)

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A. Pathogenesis of Influenza Infection (3 marks)

• Human strains: prefer alpha-2,6 sialic acid linkages (upper RT)
• Avian strains: prefer alpha-2,3 linkages (lower RT)

• Virus endocytosed; acidification of endosome activates HA (HA1-HA2 fusion)
• M2 ion channel allows H+ influx, releasing viral RNA segments into cytoplasm

• 8 ssRNA(-) segments transported to nucleus
• RNA-dependent RNA polymerase (PB1, PB2, PA) replicates viral genome
• mRNA capped using host cell cap-snatching mechanism

• New virions assembled; Neuraminidase (NA) cleaves sialic acid to allow release from host cell
• Massive release leads to destruction of ciliated epithelium

• Cytokine storm (IL-6, TNF-α, IFN) causes diffuse alveolar damage (DAD)
• DAD leads to ARDS, ground-glass opacities on CXR
• Secondary bacterial pneumonia (S. aureus, S. pneumoniae, H. influenzae) worsens prognosis

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B. Laboratory Diagnosis of Influenza (3 marks)

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C. Immunoprophylaxis (Vaccines) (3 marks)

1. Inactivated Influenza Vaccine (IIV) / Trivalent or Quadrivalent:
   • Contains killed/split/subunit virus (HA antigens of H1N1, H3N2, B strains)
   • Given IM; updated annually based on WHO surveillance
   • Suitable for all age groups including elderly, pregnant women, immunocompromised
2. Live Attenuated Influenza Vaccine (LAIV - FluMist):
   • Cold-adapted, temperature-sensitive strains; replicates only in nasal mucosa (33°C)
   • Administered intranasally
   • Not for immunocompromised, pregnant women, <2 years or >50 years
3. Recombinant Influenza Vaccine (RIV):
   • HA produced in insect cells using baculovirus system
   • No egg-based production; suitable for egg-allergic individuals

• Annual vaccination for HCWs, elderly (>65), children 6 months-5 years, pregnant women, those with chronic diseases
• Vaccine composition revised every year based on circulating strains (Northern and Southern Hemisphere formulations)

• Oseltamivir (Tamiflu) - NA inhibitor; 75 mg OD for 10 days as post-exposure prophylaxis
• Zanamivir - inhaled NA inhibitor

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D. Doctrine of "Original Antigenic Sin" (1 mark)

• On re-exposure to a new variant, memory B-cells specific to the original strain are preferentially activated
• This leads to production of antibodies that cross-react with the original antigen but are suboptimal against the new variant
• The new antibody response is inhibited/suppressed
• Named by Thomas Francis Jr. (1960)
• Clinical significance: Explains why older individuals may respond poorly to new pandemic strains; explains variable vaccine efficacy; contributes to pandemic vulnerability

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Q2. (30-year male, painless ulcer glans penis, bilateral non-tender inguinal lymphadenopathy, H/o unprotected sex)

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A. Infectious Agents Causing Sexually Transmitted Infections (STIs) (3 marks)

• Painless: Syphilis (indurated), Donovanosis
• Painful: Chancroid (H. ducreyi), Herpes (HSV)
• Painless + bilateral non-tender lymphadenopathy: Primary Syphilis (most likely)

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B. Pathogenesis & Clinical Sequelae of Syphilis (Treponema pallidum) (3 marks)

• Enters through minute abrasions in skin/mucosa during sexual contact
• Outer membrane proteins (OMPs) and lipoproteins mediate attachment to host cells

• Replicates locally → induces CD4+ T cell and macrophage response
• Endarteritis obliterans: perivascular infiltration by lymphocytes and plasma cells → ischemia

• Penetrates lymphatics → bloodstream → systemic spread within hours

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C. Specific Serological Tests for Syphilis (4 marks)

Non-Treponemal Tests (detect antibody against cardiolipin-lecithin-cholesterol antigen):

• Become positive 4-6 weeks after infection (can be negative in primary syphilis)
• Biological False Positives (BFP): SLE, RA, pregnancy, malaria, TB, leprosy, viral infections

Treponemal Tests (detect antibodies against treponemal antigens):

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SHORT ESSAYS (6 × 5 = 30 marks)

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Q3. Vaginal Candidiasis - Clinical Features & Laboratory Diagnosis

• Thick, white, curdy "cottage cheese" discharge (non-offensive)
• Intense vulvovaginal pruritus (most prominent symptom)
• Burning, dyspareunia, dysuria
• Erythema, edema of vulva and vaginal walls
• White plaques adherent to mucosa (bleeds when scraped)

1. Gram Stain of vaginal discharge:
   • Gram-positive budding yeast cells (2-4 µm)
   • Pseudohyphae and hyphae (elongated budding cells)
   • Diagnostic: Gram+ ovoid yeast + pseudohyphae
2. KOH (10-20%) Mount (Wet preparation):
   • Rapid, simple bedside test
   • Dissolves epithelial cells; reveals budding yeast + pseudohyphae
   • Sensitivity ~60-70%
3. Culture (Sabouraud's Dextrose Agar - SDA):
   • White, creamy, opaque colonies at 25-37°C within 24-48 hours
   • "Yeasty odor"
   • Blood agar: white colonies
4. Germ tube test (Reynolds-Braude phenomenon):
   • Inoculate C. albicans in serum at 37°C for 2-3 hours
   • Formation of short, unbranched tubular outgrowths (germ tubes) - POSITIVE for C. albicans (and C. dubliniensis)
5. Chlamydospore formation on Cornmeal agar:
   • C. albicans produces thick-walled chlamydospores - confirmatory
6. CHROMagar Candida:
   • Differential medium: C. albicans = green, C. tropicalis = blue, C. krusei = pink/dry
7. Molecular: PCR for species identification in recurrent/resistant cases
8. Antifungal susceptibility: For recurrent/treatment-resistant cases (CLSI method)

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Q4. Human Anthrax - Clinical Features & Laboratory Diagnosis

1. Specimen: Wound swab/vesicle fluid (cutaneous), BAL/sputum (pulmonary), blood, CSF, stool
2. Direct Smear:
   • Gram stain: Large Gram+, rectangular-ended rods in "bamboo cane/McFadyean chains"
   • Spores: oval, central-subterminal, non-bulging
   • Capsule: McFadyean reaction (polychrome methylene blue) - capsule stains pink-purple
3. Culture:
   • Blood agar: flat, irregular, "Medusa head" or "comma" colonies; ground glass appearance; non-hemolytic (differentiates from B. cereus which is hemolytic)
   • Gelatin stab: "inverted fir tree" appearance
4. Biochemical: Catalase positive, non-motile (unlike other Bacillus spp.), no hemolysis
5. Special Tests:
   • String of pearls test: Growth on penicillin-containing medium - bacteria swell into spheres; positive for B. anthracis
   • Ascoli precipitin test: Thermoprecipitation test on animal hides/meat/soil; uses anthrax antiserum
6. Serology: ELISA for anti-protective antigen antibodies
7. PCR: Detection of pXO1 (toxin genes) and pXO2 (capsule gene) plasmids - confirmatory
8. Animal inoculation: Guinea pig - death within 24-48 hours; post-mortem: blood-stained serous fluid, spleen hyperemic

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Q5. Human Rabies - Pathogenesis & Laboratory Diagnosis

1. Inoculation: Virus introduced through bite of rabid animal (dog, bat, fox)
2. Local replication: Replicates in striated muscle at bite site for days to weeks
3. Peripheral nerve entry: G-protein binds nAChR at neuromuscular junction; virus travels centripetally (retrograde axoplasmic transport) at 12-24 mm/day
4. CNS invasion: Reaches spinal cord → brainstem → limbic system → cerebral cortex
5. Dissemination: Travels centrifugally to salivary glands, cornea, skin, adrenals
6. Pathology in CNS:
   • Encephalitis - lymphocytic perivascular cuffing
   • Negri bodies - intracytoplasmic eosinophilic inclusion bodies in hippocampal pyramidal cells (Sommer's sector) and Purkinje cells of cerebellum - PATHOGNOMONIC
7. Clinical Disease:
   • Furious (encephalitic) rabies (80%): Hydrophobia (pharyngeal spasm on seeing water), aerophobia, hypersalivation, agitation, hallucinations
   • Dumb (paralytic) rabies (20%): Ascending paralysis mimicking Guillain-Barre syndrome

• Wound washing with soap/water + antiseptic
• RIG (Rabies Immunoglobulin) at wound site
• 4-dose vaccine schedule (Essen regimen): Days 0, 3, 7, 14

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Q6. Primary Amoebic Meningoencephalitis (PAM) - Pathogenesis & Lab Diagnosis

1. Entry: Trophozoites enter nasal mucosa during swimming/diving in warm freshwater
2. Nasal penetration: Cross the cribriform plate of ethmoid bone via olfactory nerve endings
3. CNS invasion: Trophozoites invade olfactory bulbs and frontal lobes directly
4. Tissue destruction: Trophozoites secrete:
   • Phospholipases and proteases - lyse cells
   • Nfa1 protein (food cup/amoebostome) - contact-dependent killing
   • Reactive oxygen species → hemorrhagic necrotic encephalitis
5. Inflammation: Purulent meningitis; neutrophilic pleocytosis in CSF
6. Clinical features: Acute, rapidly fatal (death in 3-7 days):
   • Sudden onset severe headache, fever, nausea
   • Olfactory symptoms (anosmia, altered taste) - early clue
   • Meningismus, altered consciousness
   • No amoeba in blood; CSF: high pressure, neutrophils, elevated protein, low glucose, RBCs
   • Mortality >97%

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Q7. Laboratory Diagnosis of Bacterial Meningitis

• Neonates: E. coli, Group B Streptococcus, L. monocytogenes
• Children: N. meningitidis, S. pneumoniae, H. influenzae type b
• Adults/Elderly: S. pneumoniae, N. meningitidis, L. monocytogenes

1. Gram Stain of CSF (urgent):
   • S. pneumoniae: Gram+ diplococci (lancet-shaped)
   • N. meningitidis: Gram- intracellular diplococci
   • H. influenzae: Gram- coccobacilli
   • E. coli: Gram- rods
   • Sensitivity: ~60-90% if untreated; decreases after antibiotics
2. Culture & Sensitivity:
   • Blood agar, Chocolate agar, MacConkey agar
   • Incubate at 37°C, 5% CO₂ (for fastidious organisms)
   • Blood cultures (2 sets before antibiotics)
   • Sensitivity ~60-70%
3. Antigen Detection (Latex Agglutination):
   • Detects capsular polysaccharide antigens of S. pneumoniae, N. meningitidis (A/C/Y/W), H. influenzae b, Group B Streptococcus, E. coli K1
   • Useful after antibiotic treatment (when culture may be negative)
4. Molecular Tests:
   • PCR (multiplex): BioFire FilmArray Meningitis/Encephalitis Panel - detects 14 pathogens simultaneously in CSF
   • High sensitivity, rapid results (1-2 hours)
5. India Ink stain: Rules out Cryptococcus neoformans (large capsule)
6. Other rapid tests:
   • Procalcitonin (serum) - elevated in bacterial meningitis
   • CRP - markedly elevated
   • Serum lactate

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Q8. Brucellosis - Clinical Features & Laboratory Diagnosis

• Incubation: 1-6 weeks
• Acute/Subacute: Undulant fever (fever rises in evening, falls in morning - "undulating"), profuse night sweats, malaise, myalgia, arthralgia, headache, anorexia
• Focal complications: Sacroiliitis, spondylitis (lumbar spine), orchitis (most common in males), hepatosplenomegaly, endocarditis (rare but most fatal)
• Chronic (>1 year): Depression, fatigue, arthropathy; difficult to distinguish from psychosomatic illness
• Relapse is common

1. Blood culture (gold standard):
   • BACTEC/BACT-Alert automated system
   • Slow-growing: incubate up to 6 weeks (long incubation)
   • Castañeda biphasic bottle (traditional method): blood + liquid + solid medium
   • BSL-3 organism
2. Bone marrow culture:
   • Higher sensitivity than blood culture (especially after antibiotics)
3. Gram stain: Tiny Gram-negative coccobacilli (rarely seen due to fastidious nature)
4. Serology (most commonly used in clinical practice):

   Test	Details
   Standard Agglutination Test (SAT/Wright's test)	Tube agglutination; serum + B. abortus antigen; titer ≥1:160 significant (endemic areas); detects IgM (early), IgG (chronic)
   Rose Bengal Test (RBT)	Rapid card agglutination; B. abortus antigen stained with Rose Bengal dye at pH 3.6 (detects IgG); screening test
   2-ME test (2-Mercaptoethanol)	Adds 2-ME to SAT; destroys IgM; persistent agglutination indicates IgG (chronic/active disease); distinguishes acute (IgM) from chronic (IgG)
   Coombs test	Detects non-agglutinating (incomplete) antibodies; useful when SAT negative despite symptoms
   ELISA	Sensitive; IgG, IgM, IgA separately; useful for diagnosis and follow-up
   Brucellacapt	Single tube immunocapture agglutination test

5. PCR: Real-time PCR on blood/tissue; highly sensitive and specific; useful in seronegative cases and post-treatment follow-up
6. Other: Mild leukopenia with relative lymphocytosis; elevated ESR, CRP

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SHORT ANSWERS (10 × 3 = 30 marks)

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Q9. ASO Test (Anti-Streptolysin O Test)

• Principle: Measures antibodies (IgG) against Streptolysin O (oxygen-labile exotoxin of Group A Streptococcus pyogenes)
• Method: Tube dilution neutralization - serial dilutions of patient's serum incubated with streptolysin O; add washed RBCs; the highest dilution that prevents hemolysis = ASO titer; expressed in Todd units
• Significance:
  • Evidence of recent streptococcal infection
  • Elevated in: Rheumatic fever (>200 Todd units in adults, >330 in children), post-streptococcal glomerulonephritis
  • Not elevated in uncomplicated streptococcal skin infections (pyoderma)
• Rapid latex test (RAST): Latex particles coated with streptolysin O agglutinate with patient antibodies
• Normal: <200 Todd units (adults); pediatric values slightly higher
• Note: DNase B antibody test is better for skin infections

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Q10. COVID-19 Prophylaxis

• Mask wearing (N95/surgical), hand hygiene, physical distancing, ventilation
• Isolation of infected individuals; contact tracing

• Primary vaccination series + booster doses
• Target groups for priority: HCWs, elderly, immunocompromised, comorbidities

• Antivirals: Paxlovid (nirmatrelvir + ritonavir), Remdesivir (hospitalized)
• Monoclonal antibodies: Bebtelovimab, Evusheld (tixagevimab + cilgavimab) - for pre-exposure prophylaxis in immunocompromised

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Q11. Aspergilloma

• Definition: A fungal ball ("mycetoma") formed by Aspergillus fumigatus (most common) growing within a pre-existing lung cavity
• Pre-existing cavity causes: Old TB cavities (most common in India), pulmonary sarcoidosis, bronchiectasis, lung abscess, emphysematous bullae
• Pathogenesis: Fungal hyphae, mucus, fibrin, cellular debris, and necrotic material form a ball within the cavity; hyphal invasion is absent (unlike invasive aspergillosis)
• Clinical features: May be asymptomatic; hemoptysis (can be massive, life-threatening - due to erosion of blood vessels); cough, dyspnea
• Radiology: CXR/CT - Monod sign (air crescent sign): radiopaque ball with air crescent above it; ball moves with change in posture
• Diagnosis:
  • CXR/HRCT chest
  • Sputum culture: Aspergillus spp. (septate hyphae, V-shaped/45° angle branching)
  • Serology: Precipitating antibodies (IgG precipitins) against Aspergillus antigens - positive in >90%
  • Serum Galactomannan (less useful than in invasive aspergillosis)
• Treatment: Surgical resection (cavernostomy/lobectomy) if symptomatic; systemic antifungals have limited role; bronchial artery embolization for hemoptysis

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Q12. Endemic Haematuria

• Definition: Haematuria endemic in areas with heavy Schistosoma haematobium infection; terminal haematuria (blood at end of micturition)
• Causative organism: Schistosoma haematobium (blood fluke) - trematode
• Transmission: Cercariae penetrate intact skin during contact with infected freshwater (snail: Bulinus spp.); endemic in Africa, Middle East
• Pathogenesis:
  • Adult worms inhabit vesical (pelvic) venous plexus
  • Females lay eggs with terminal spines → eggs penetrate bladder wall → cause intense granulomatous inflammation → fibrosis, calcification
• Clinical features:
  • Terminal haematuria (painless, classic)
  • Dysuria, frequency
  • "Sandy patches" on cystoscopy (calcified eggs)
  • "Cobblestone bladder" / "Egg shell calcification" on X-ray
  • Obstructive uropathy, hydronephrosis
  • Complications: Squamous cell carcinoma of bladder (chronic infection), secondary bacterial infections
• Diagnosis:
  • Urine microscopy (terminal urine): Eggs with terminal spine
  • Rectal snip: Eggs in tissue
  • Serological tests: ELISA
  • Cystoscopy, biopsy
• Treatment: Praziquantel (drug of choice) - single dose 40 mg/kg

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Q13. Pneumococcal Vaccines

• Contains purified capsular polysaccharides from 23 serotypes
• T-cell independent response (IgM predominant); no immunological memory
• Not effective in children <2 years
• Given to adults >65 years, immunocompromised, asplenic patients, chronic diseases

• Polysaccharides conjugated to carrier protein (CRM197 - non-toxic diphtheria toxoid)
• T-cell dependent response; induces immunological memory; mucosal IgA
• Effective from 6 weeks of age
• Recommended in India's UIP (Universal Immunization Programme): PCV at 6 weeks, 10 weeks, 14 weeks, 9-12 months booster
• Covers serotypes responsible for most pediatric IPD (invasive pneumococcal disease)

• PPSV23 covers 23 serotypes (including 1, 3, 5, 6A, 6B, 9V, 14, 19A, 19F, 23F)
• PCV13 covers 13 serotypes

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Q14. Kissing Disease (Infectious Mononucleosis)

• Causative agent: Epstein-Barr Virus (EBV) - Human herpesvirus 4 (HHV-4); dsDNA enveloped virus
• Transmission: Saliva ("Kissing disease"), blood, rarely organ transplant; primarily in adolescents/young adults
• Pathogenesis:
  • EBV infects oropharyngeal epithelium → infects B lymphocytes via CD21 (CR2, complement receptor) binding gp350
  • Polyclonal B cell activation → atypical lymphocytosis (reactive CD8+ T cells/NK cells = Downey cells)
  • EBV establishes latent infection in B cells (lifetime)
• Clinical Features (Classic Triad):
  • Fever
  • Pharyngotonsillitis (exudative, severe)
  • Lymphadenopathy (posterior cervical most characteristic)
  • • Splenomegaly, hepatitis, fatigue, rash (esp. with amoxicillin - maculopapular rash)
• Laboratory Diagnosis:
  • Blood picture: Leukocytosis; >10% atypical lymphocytes (Downey cells - large, blue, indented nucleus)
  • Paul-Bunnell test (Heterophile antibody test): Patient's serum agglutinates sheep RBCs; absorbed with guinea pig kidney (removes non-specific Forssman antibodies); positive titer ≥1:112; specificity improved by absorption
  • Monospot test (rapid): Horse RBCs used; simpler, quicker version of Paul-Bunnell
  • EBV-specific antibodies (VCA-IgM, EA, EBNA): Gold standard for diagnosis and staging
  • LFTs: Mildly elevated transaminases
• Complications: Splenic rupture, airway obstruction, Burkitt's lymphoma, nasopharyngeal carcinoma (long-term)

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Q15. Lab Diagnosis of CNS Cryptococcosis

• Causative agent: Cryptococcus neoformans (type A - most common in immunocompromised) and C. gattii (immunocompetent)
• Population at risk: HIV/AIDS (CD4 <100 cells/µL), organ transplant, hematological malignancies

• High opening pressure (hallmark)
• Lymphocytic pleocytosis (low cell count, sometimes near-normal)
• Elevated protein, low glucose
• Yeast cells visible

1. India Ink preparation of CSF (most classic):
   • Large encapsulated yeast cells surrounded by halo (polysaccharide capsule excludes ink)
   • Single budding yeast with large capsule ("Chinese lantern" or "halo" appearance)
   • Sensitivity: ~50-80% in HIV patients
2. Cryptococcal Antigen (CrAg) test (most sensitive):
   • Latex Agglutination (LA) or Lateral Flow Assay (LFA)
   • Detects capsular polysaccharide (glucuronoxylomannan - GXM)
   • Serum or CSF; sensitivity >95% in HIV patients
   • Can be positive weeks before symptoms
   • LFA: Can be performed on urine/serum too; used for screening in Africa
3. Culture:
   • SDA, Bird seed agar (niger seed agar/Staib agar)
   • Bird seed agar: Cryptococcus produces brown pigment colonies (phenol oxidase enzyme converts caffeic acid → melanin)
   • Mucoid, cream-colored colonies
4. Stains:
   • Mucicarmine stain: Capsule stains red/pink
   • PAS stain: Capsule stains pink
   • Alcian Blue: Capsule stains blue
5. Serology: Serum CrAg titer correlates with fungal burden; used to monitor treatment response
6. Molecular: PCR (research use)

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Q16. Hydatid Cyst (Labelled Diagram)

         ┌─────────────────────────────────────┐
         │         HOST PERICYST               │ ← Host-derived fibrous layer (adventitia)
         │   ┌─────────────────────────────┐   │
         │   │    ECTOCYST (outer layer)   │   │ ← Laminated membrane (acellular, white, chitinous)
         │   │  ┌───────────────────────┐  │   │
         │   │  │  ENDOCYST (inner)     │  │   │ ← Germinal/Proliger layer (nucleated, living)
         │   │  │                       │  │   │
         │   │  │  ○○ Brood capsules    │  │   │ ← Contains scolices (protoscolices)
         │   │  │  ╔═╗ Daughter cysts   │  │   │
         │   │  │     Hydatid sand      │  │   │ ← Free scolices + calcareous corpuscles
         │   │  │     (scolices+hooks)  │  │   │
         │   │  └───────────────────────┘  │   │
         │   │          Hydatid fluid      │   │ ← Clear, colorless; antigenic
         │   └─────────────────────────────┘   │
         └─────────────────────────────────────┘

1. Pericyst (host-derived fibrous adventitia) - outermost
2. Ectocyst / Laminated membrane (non-nucleated, white, multi-laminar)
3. Endocyst / Germinal layer (inner nucleated layer - produces all other structures)
4. Brood capsules (contain protoscolices with 4 suckers + 2 rows of hooks)
5. Daughter cysts (secondary cysts within)
6. Hydatid fluid (clear fluid containing hydatid sand)
7. Hydatid sand (detached scolices, hooklets, calcareous corpuscles)

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Q17. Human Prion Disease

• Non-conventional infectious agents - contain no nucleic acid (DNA or RNA)
• Composed entirely of abnormally folded prion protein: PrP^Sc (scrapie form, beta-sheet conformation) vs. normal PrP^C (alpha-helix conformation)
• Mechanism: PrP^Sc acts as a template, converting normal PrP^C → PrP^Sc (autocatalytic cascade)
• Highly resistant to heat, UV, formalin, nucleases; inactivated by NaOH, autoclaving at 134°C

• EEG: Periodic sharp wave complexes (PSWCs)
• MRI: Diffusion-weighted imaging (DWI) - cortical ribboning, basal ganglia signal
• CSF: 14-3-3 protein (marker of neuronal death); RT-QuIC (real-time quaking-induced conversion) - highly sensitive
• Brain biopsy (gold standard): Spongiform change, astrogliosis, PrP deposits

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Q18. Autonomy (Medical Ethics)

1. Autonomy
2. Beneficence
3. Non-maleficence
4. Justice

1. Informational autonomy: Right to receive full, honest information about diagnosis, treatment options, risks, and prognosis
2. Decisional autonomy: Right to make decisions without coercion
3. Competence: Patient must have capacity to understand, reason, and communicate choices

• Disclosure of relevant information
• Patient comprehension
• Voluntary decision-making
• Competence/capacity of the patient

• Emergency situations: When patient cannot consent
• Minors: Parents/guardians decide (assent obtained from child)
• Mental incapacity: Substitute decision-maker/advanced directive
• Public health threat: Quarantine/isolation may override individual autonomy
• Autonomy does not compel a clinician to provide futile or harmful treatment

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Exam Tips:

• Q1: The pathogen is Influenza virus (ARDS in HCW + shift & drift = classic scenario)
• Q2: Painless indurated ulcer + non-tender lymphadenopathy = Primary Syphilis (T. pallidum); the picture shows spirochetes (dark field) and a genital chancre
• For diagram questions (Q16), always draw neatly with 5+ labels
• Short answers: aim for 3 key points each = full 3 marks