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✅ COMPLETE EXAM SOLUTION — Microbiology Pre-University Paper II (80 Marks)

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SECTION I — LONG ANSWER QUESTIONS (10×2 = 20 Marks)

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Q1. Painless penile ulcer with rubbery inguinal lymph nodes — (10 Marks)

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(i) Probable Diagnosis & Etiological Agent — 2 Marks

• A spirochete (corkscrew-shaped organism)
• Gram-negative, NOT visible on Gram stain — requires dark-field microscopy

Key clinical clue: Painless indurated (hard-based) ulcer + non-tender rubbery lymphadenopathy = classic primary syphilitic chancre

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(ii) Pathogenesis — 4 Marks

1. Entry: T. pallidum enters through micro-abrasions in mucosa/skin during sexual contact
2. Local multiplication: Organism multiplies at the site → triggers inflammatory response → forms the primary chancre (indurated, painless ulcer) within 10–90 days (average 3 weeks)
3. Lymphatic spread: Organisms travel to regional lymph nodes → bilateral painless rubbery lymphadenopathy (bubo)
4. Immune evasion: T. pallidum has very few surface proteins → poor immunogenicity → evades immune detection
5. Dissemination (Secondary stage if untreated): Enters bloodstream → spreads systemically → skin rash on palms/soles, condylomata lata, mucous patches
6. Latent stage: Immune system partially controls → asymptomatic period
7. Tertiary stage: Gumma formation, cardiovascular syphilis (aortitis), neurosyphilis (tabes dorsalis, general paresis)

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(iii) Laboratory Diagnosis & Treatment — 4 Marks

Important: VDRL positive → confirm with FTA-ABS. VDRL can be false positive in SLE, malaria, pregnancy.

• Drug of choice: Benzathine Penicillin G — 2.4 million units IM single dose (primary/secondary)
• If penicillin allergic: Doxycycline 100 mg BD × 14 days OR Azithromycin
• Partner notification and treatment is mandatory

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Q2. 25-year-old with dog bite — (10 Marks)

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(i) Pathogenesis & Laboratory Diagnosis of Rabies Virus — 5 Marks

1. Entry: Virus inoculated into wound via infected saliva during bite
2. Local replication: Replicates in muscle cells at bite site (incubation: 1–3 months, but can range 1 week to 1 year)
3. Peripheral nerve invasion: Binds nicotinic acetylcholine receptors (nAChR) → enters peripheral nerve endings → travels via retrograde axonal transport towards CNS
4. CNS involvement: Reaches spinal cord and brain → multiplies in neurons → causes encephalitis
5. Centrifugal spread: Travels outward from CNS to salivary glands, cornea, skin → explains viral shedding in saliva
6. Clinical features: Prodrome (fever, pain at bite site) → Furious form (hydrophobia, aerophobia, agitation) OR Dumb form (ascending paralysis) → Coma → Death
7. Hallmark: Negri bodies = eosinophilic intracytoplasmic inclusions in Purkinje cells of cerebellum and hippocampal neurons

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(ii) Immunization & Prophylactic Measures for Dog Bite — 5 Marks

• For high-risk groups (veterinarians, lab workers)
• HDCV (Human Diploid Cell Vaccine) or PCECV — Days 0, 7, 21/28
• Booster every 2 years

• Wash wound vigorously with soap and water for ≥15 minutes
• Apply antiseptic (povidone-iodine, alcohol)
• Do NOT suture the wound immediately
• Tetanus prophylaxis if needed

• Intramuscular regimen (Essen): Days 0, 3, 7, 14, 28 (5 doses)
• Intradermal regimen (ID): 2 sites × 0.1 ml on Days 0, 3, 7, 28 (2-site ID protocol — WHO approved, cost-saving)
• Vaccines: HDCV, PCECV, Purified Vero Cell Vaccine (PVRV)

• Human RIG (HRIG): 20 IU/kg — preferred
• Equine RIG (ERIG): 40 IU/kg — after skin test
• Infiltrate maximum into and around the wound, remainder IM at distant site
• Given on Day 0 only (do NOT repeat)

Key rule: If animal is healthy and can be observed for 10 days, PEP can be deferred until animal shows signs.

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SECTION II — EXPLAIN THE FOLLOWING (3×5 = 15 Marks)

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1. MRSA resistant to Penicillin — Why? (3 Marks)

• Beta-lactams bind to Penicillin Binding Proteins (PBPs) (transpeptidases) on bacterial cell wall → inhibit peptidoglycan cross-linking → bactericidal

1. Acquisition of mecA gene — carried on a mobile genetic element called Staphylococcal Cassette Chromosome mec (SCCmec)
2. mecA gene encodes a novel PBP called PBP2a (or PBP2')
3. PBP2a has very low affinity for all beta-lactam antibiotics (including methicillin, oxacillin, all penicillins, cephalosporins, carbapenems)
4. Cell wall synthesis continues using PBP2a even in the presence of beta-lactams → resistance

• Beta-lactamase production (destroys penicillin ring — resistance to natural penicillins)
• Efflux pumps and altered outer membrane permeability

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2. Quellung Reaction by Streptococcus pneumoniae — Why? (3 Marks)

1. S. pneumoniae has a prominent polysaccharide capsule (major virulence factor — anti-phagocytic)
2. When type-specific anti-capsular antibodies (or polyvalent antisera) are mixed with the organism:
3. Antibodies bind to capsular polysaccharide antigens → antigen-antibody complex forms on the capsule surface
4. This complex increases refractive index of the capsule → capsule appears swollen, refractile (bright), and clearly demarcated under light microscopy
5. The capsule does NOT actually swell (a misnomer) — it only becomes more visible

• Used for capsular serotyping — identifies the specific pneumococcal serotype (84 serotypes)
• Diagnostic test for S. pneumoniae
• Capsule is tested using Quelling reaction, India ink stain (for Cryptococcus too)

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3. Cytokine Storm in SARS-CoV-2 — Why? (3 Marks)

• Virus enters via ACE-2 receptor (lungs, heart, kidneys) → innate immune response activated → cytokines released to fight infection

1. Massive uncontrolled activation of innate immune cells (macrophages, neutrophils, NK cells, T cells)
2. Enormous release of pro-inflammatory cytokines: IL-6, IL-1β, IL-8, TNF-α, IFN-γ
3. This "storm" causes more damage than the virus itself:
   • Diffuse alveolar damage (ARDS)
   • Vascular leak → edema
   • Multi-organ failure (lungs, kidneys, heart)
4. Why it happens: Delayed type I IFN response → unchecked early viral replication → massive late immune activation
5. Certain patient groups (elderly, obese, diabetics) have baseline dysregulated immunity → more prone

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4. Trismus characteristically seen in Tetanus — Why? (3 Marks)

1. C. tetani spores enter wound → germinate in anaerobic conditions → produce tetanospasmin
2. Toxin travels via retrograde axonal transport to CNS
3. Mechanism of toxin: Tetanospasmin is a zinc-dependent metalloprotease that cleaves synaptobrevin (VAMP — a SNARE protein)
4. This blocks release of inhibitory neurotransmitters — GABA and Glycine — from inhibitory interneurons (Renshaw cells) in the spinal cord
5. Result: Unopposed excitatory motor neuron activity → sustained muscle spasms

• Masseter muscle (jaw muscle) receives the shortest path from CNS → affected earliest
• Trismus = inability to open mouth (lockjaw) = first clinical sign of tetanus
• Followed by: Risus sardonicus (sardonic smile), opisthotonos (arching of back), generalized convulsions

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5. Pulse Polio Immunization useful to eradicate Polio — How? (3 Marks)

1. Interrupts transmission: Administering OPV to ALL children <5 years on a single day → massive simultaneous immunization → no susceptible child remains → virus cannot transmit
2. Herd immunity: Even unimmunized children are protected when enough population is immune (threshold ~85–90%)
3. Gut immunity: OPV produces local secretory IgA in gut mucosa → prevents intestinal replication and fecal-oral transmission (wild poliovirus spreads via fecal-oral route) — advantage over IPV
4. Contact immunization: OPV virus sheds in stools → immunizes unvaccinated contacts in community (especially important in low-sanitation areas)
5. Strategy: Two rounds of National Immunization Days (NIDs) + Sub-NIDs — administered 4–6 weeks apart to "catch" children who missed the first round
6. India declared polio-free in 2014 — thanks to this strategy

Advantage of OPV over IPV here: OPV produces gut immunity + contact spread → better at interrupting wild virus transmission in developing countries

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SECTION III — APPLIED QUESTIONS (5×4 = 20 Marks)

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Q1. Road Traffic Accident — Black, foul-smelling wound discharge (4 Marks)

• Gram-positive, spore-forming, anaerobic, non-motile
• Produces lecithinase (alpha toxin) — major virulence factor → destroys cell membranes

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Q2. Fever + Coated tongue + Rose spots — Typhoid (4 Marks)

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Q3. Pus cells in urine + Lactose fermenting colonies — UTI (4 Marks)

1. Urine microscopy: >10 WBCs/HPF, bacteria, >10⁵ organisms/mL
2. Dipstick: Leukocyte esterase positive, nitrite positive
3. Culture on CLED agar (Cystine-Lactose-Electrolyte-Deficient) / MacConkey

• E. coli: Pink lactose-fermenting colonies on MacConkey
• Flat yellow colonies on CLED

1. Colony count: Standard loop = 1µL → count colonies × 1000 = CFU/mL
2. IMViC reactions for E. coli: Indole +, MR +, VP −, Citrate −
3. Antibiotic sensitivity testing (AST) — Kirby-Bauer disc diffusion

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Q4. Multiple sex partners + Unexplained fever + Weight loss + Diarrhea + Lymphadenopathy — 6 months (4 Marks)

1. Virus: HIV-1 (common), HIV-2 — Retrovirus, ssRNA, has reverse transcriptase
2. Entry: Virus binds CD4 receptor + co-receptor (CCR5 or CXCR4) on CD4+ T lymphocytes, macrophages, dendritic cells
3. Replication: Reverse transcriptase converts RNA → DNA → integrates into host genome as provirus
4. CD4 cell destruction: Viral replication → cell lysis, immune-mediated killing → progressive fall in CD4 count
5. Immunodeficiency: CD4 count falls below critical thresholds → susceptibility to opportunistic infections

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SECTION IV — SHORT ANSWER QUESTIONS (5×5 = 25 Marks)

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1. Laboratory Diagnosis of Pulmonary Tuberculosis (5 Marks)

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2. Medico-Legal Aspects of Confidentiality (5 Marks)

• Indian Medical Council (Professional Conduct, Etiquette and Ethics) Regulations, 2002
• Section 269/270 IPC (spreading infection)
• Consumer Protection Act

• Patient's medical history, diagnosis, treatment
• Information shared during consultation
• Even after patient's death

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3. Food Poisoning (5 Marks)

• Preformed toxin in canned foods
• Blocks ACh release at NMJ → descending flaccid paralysis
• Neurotoxin (types A, B, E most common)
• Treatment: Trivalent antitoxin (ABE), supportive

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4. Candidiasis (5 Marks)

• Normally commensal in mouth, gut, vagina
• Becomes pathogenic in immunocompromised hosts

• Superficial: Topical clotrimazole, nystatin, fluconazole
• Systemic/Invasive: IV fluconazole, amphotericin B, echinocandins (caspofungin)

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5. Cryptococcal Meningitis (5 Marks)

• Large polysaccharide capsule → anti-phagocytic → major virulence factor
• Melanin production → scavenges free radicals → resists oxidative killing
• CNS tropism — grows in CNS due to presence of specific nutrients

• Induction: Amphotericin B + 5-Flucytosine × 2 weeks
• Consolidation: Fluconazole 400 mg/day × 8 weeks
• Maintenance: Fluconazole 200 mg/day (lifelong in HIV, until CD4 >200)
• Management of raised ICP: Serial lumbar puncture, acetazolamide

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📝 QUICK EXAM TIPS FOR THIS PAPER

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✅ MICROBIOLOGY PRE-UNIVERSITY EXAM — PAPER II (80 Marks)

Date: 24.07.2026 | Time: 2 hrs 30 min

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SECTION I — STRUCTURED LONG ANSWER (10×2 = 20 Marks)

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Q1. Rice-water diarrhea + Severe dehydration + Actively motile organism with darting motility (10 Marks)

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a) Disease and Causative Pathogen — 2 Marks

• Gram-negative, curved comma-shaped rod
• Highly motile — single polar flagellum → characteristic "darting/shooting star" motility seen on dark-field/phase contrast microscopy
• Facultative anaerobe; oxidase positive

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b) Pathogenesis & Laboratory Diagnosis — 3+3 = 6 Marks

Pathogenesis:

1. Ingestion: V. cholerae enters via contaminated water/food (fecal-oral route); infectious dose ~10⁸ organisms (reduced by antacids/achlorhydria)
2. Colonization: Organism passes stomach → reaches small intestine → attaches to epithelial cells via toxin co-regulated pili (TCP) and adhesins
3. Toxin production: Bacteria secrete Cholera Toxin (CT) — an AB-type enterotoxin

• B subunit (binding): 5 B subunits bind to GM1 ganglioside receptors on intestinal epithelial cells
• A subunit (active): Enters cell → ADP-ribosylates Gs-alpha protein → permanently activates adenylyl cyclase

1. cAMP elevation: Markedly elevated intracellular cAMP → activates protein kinase A
2. Net effect on ion transport:

• Inhibits NaCl absorption (blocks Na⁺/Cl⁻ co-transport)
• Stimulates Cl⁻ secretion into intestinal lumen
• Water follows osmotically → massive secretory (non-inflammatory) diarrhea

1. Clinical result:

• Profuse painless watery diarrhea — "rice water stool" (grey, turbid, with mucus flecks — no blood, no pus)
• Vomiting (early)
• Rapid severe dehydration → hypovolemic shock → death if untreated

Key: Cholera is a pure secretory diarrhea — NO mucosal invasion, NO inflammation, NO fever typically

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Laboratory Diagnosis:

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c) Prophylaxis — 2 Marks

• Safe drinking water (boil/chlorinate/filter)
• Proper sewage disposal and sanitation
• Handwashing with soap
• Safe food handling; avoid street food

• Oral Rehydration Therapy (ORS) — mainstay of treatment
• IV fluids (Ringer's Lactate) for severe dehydration
• Antibiotics (Doxycycline/Azithromycin) — shorten duration, reduce shedding

• Oral Cholera Vaccine (OCV):
• Shanchol / Dukoral — killed whole-cell oral vaccines
• 2 doses, 2 weeks apart
• Protection: ~65% for 2–3 years
• WHO recommends for endemic areas and outbreaks

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Q2. Jaundice + Hepatomegaly + History of blood transfusion + HBsAg positive (10 Marks)

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a) Most Probable Etiological Diagnosis — 1 Mark

• HBV = Hepadnavirus; partially double-stranded circular DNA virus
• Contains: HBsAg (surface), HBcAg (core), HBeAg (secreted — marker of active replication)

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b) Modes of Transmission — 2 Marks

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c) Viral Markers + Lab Diagnosis + Prophylaxis — 2+3+2 = 7 Marks

Viral Markers of HBV (Most important table in exam):

Window period: HBsAg cleared but Anti-HBs not yet appeared → ONLY anti-HBc IgM positive

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Laboratory Diagnosis:

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Prophylaxis:

• HBV Vaccine — recombinant HBsAg subunit vaccine
• Schedule: 0, 1, 6 months (3-dose) — standard adult
• Neonates: Birth, 6 weeks, 10 weeks, 14 weeks (EPI schedule in India)
• Anti-HBs titre >10 mIU/mL = protective
• High-risk groups: HCWs, dialysis patients, sex workers, household contacts of HBV carriers

• HBIG (Hepatitis B Immunoglobulin) — 0.06 mL/kg IM — passive immunity — given within 24 hrs
• HBV vaccine simultaneously at different site

• Tenofovir (TDF) or Entecavir — nucleoside/nucleotide analogues
• Pegylated Interferon-alpha (Peg-IFN-α) — immune modulator

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SECTION II — SHORT NOTES (REASONING) (5×3 = 15 Marks)

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1. Quellung Reaction by Streptococcus pneumoniae — Why? (3 Marks)

1. S. pneumoniae possesses a prominent polysaccharide capsule — its key virulence factor (anti-phagocytic)
2. When type-specific anti-capsular antibodies (Neufeld's antisera) are mixed with the organism on a slide:
3. Antibodies bind to capsular polysaccharides → form antigen-antibody complex on capsule surface
4. This increases the optical refractive index of the capsule → capsule appears swollen, highly refractile, and sharply demarcated under light microscopy (capsule does NOT actually swell — it only becomes more visible)

• Used for serotyping S. pneumoniae (84 serotypes identified)
• Diagnostic identification of pneumococcus
• First described by Neufeld → also called Neufeld's reaction

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2. PCR Preferred for Diagnosing Viral Infections — Why? (3 Marks)

1. Extreme sensitivity: Can detect as few as 1–10 copies of viral genome — detects virus even at very low levels (below detection threshold of culture or serology)
2. Speed: Results in hours vs. days/weeks for viral culture
3. Detects non-viable/non-culturable viruses: Works even with dead virus, degraded samples, or viruses that don't grow in culture (e.g., Norovirus, HPV, hepatitis viruses)
4. Window period advantage: Detects viral nucleic acid before antibodies appear (e.g., HIV RNA detected at 11 days vs. ELISA at 3–4 weeks)
5. Quantitative (RT-qPCR): Gives viral load → monitors disease progression and treatment response (e.g., HIV viral load, HBV DNA, CMV)
6. Multiplex PCR: Detects multiple pathogens simultaneously from one sample
7. High specificity: Using viral-specific primers → no cross-reactivity with host genome

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3. BCG Not Given to Immunocompromised Children — Why? (3 Marks)

1. Live vaccine can cause disease: In an immunocompetent child, attenuated BCG is controlled by normal cellular immunity (T-cell mediated)
2. In immunocompromised children (HIV/AIDS, SCID, DiGeorge syndrome, on immunosuppressants) — cellular immunity is severely deficient
3. BCG organisms replicate unchecked → cause BCG-osis / disseminated BCG disease — local ulceration, regional lymphadenitis, systemic dissemination to liver, spleen, bone, CNS → potentially fatal
4. WHO recommendation: BCG is contraindicated in:

• Symptomatic HIV-infected children
• SCID, CGD, other primary immunodeficiencies
• Children on high-dose steroids/immunosuppressants
• Malignancy

In India: Asymptomatic HIV-exposed neonates may receive BCG, but symptomatic HIV-infected children should NOT.

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4. Empirical Antibiotics Must Be Started Immediately in Suspected Meningitis — Why? (3 Marks)

1. Life-threatening condition: Bacterial meningitis (especially N. meningitidis, S. pneumoniae, Listeria) has a mortality rate of 20–30% — can kill within hours if untreated
2. Rapid neurological damage: Every hour of delay in antibiotic therapy increases mortality and risk of permanent sequelae (deafness, brain damage, seizures)
3. Diagnosis takes time: Lumbar puncture results (CSF analysis, culture) may take 24–48 hours; culture sensitivity is reduced if antibiotics given first, BUT the delay risk outweighs this
4. CSF white cell count and biochemistry remain altered even after antibiotics, allowing diagnosis — and PCR/antigen tests work on pre-antibiotic CSF too
5. Empirical cover: While awaiting CSF results, empirical antibiotics cover the most likely organisms by age:

• Neonates: Ampicillin + Gentamicin/Cefotaxime
• Children/Adults: Ceftriaxone (3rd gen cephalosporin) ± Dexamethasone
• Elderly/immunocompromised: Ceftriaxone + Ampicillin (for Listeria)

1. Blood cultures should be drawn BEFORE starting antibiotics (takes only 2–3 minutes) to increase microbiological yield

Principle: "Treat first, diagnose in parallel" — never delay life-saving treatment waiting for microbiology results

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5. Pulse Polio Immunization Useful to Eradicate Polio — How? (3 Marks)

1. OPV (Oral Polio Vaccine) used = Sabin live attenuated vaccine (types 1, 2, 3)
2. Simultaneous mass immunization: Giving OPV to ALL children <5 years on one day nationwide → removes all susceptible individuals at once → virus cannot find a new host → chain of transmission broken
3. Intestinal (mucosal) immunity: OPV produces secretory IgA in gut → prevents intestinal replication of wild poliovirus → blocks fecal-oral transmission (primary route of spread)
4. Herd immunity: When >85–90% of population is immune, even unvaccinated individuals are protected
5. Contact immunization (secondary spread): OPV virus sheds in stools → spreads to and immunizes unvaccinated contacts in community (especially effective in low-sanitation areas)
6. Two rounds of NIDs (National Immunization Days) 4–6 weeks apart — "mopping up" strategy catches children missed in first round
7. India declared polio-free in 2014 using this strategy

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SECTION III — SHORT NOTES APPLIED (4×5 = 20 Marks)

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Q1. Evening fever + Night sweats + Weight loss + Chronic cough + Beaded acid-fast bacilli (5 Marks)

a) Provisional Diagnosis: Pulmonary Tuberculosis

• Acid-fast bacillus (AFB): Gram-positive, non-motile, non-sporing, obligate aerobe
• Slow-growing (doubling time 18–24 hrs)
• "Beaded" AFB on sputum = classic microscopic description

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b) Laboratory Diagnosis:

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Drug Resistance in M. tuberculosis:

• katG gene mutation → isoniazid resistance
• rpoB gene mutation → rifampicin resistance
• inhA mutation → isoniazid + ethionamide resistance
• Horizontal gene transfer NOT involved (M.tb has no plasmids) — all mutations are chromosomal, spontaneous

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Q2. High-grade fever + Vomiting + Flank pain + Increased urinary frequency — 3 days (5 Marks)

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Q3. High-grade fever + Headache + Joint pain + Petechiae + Low platelet count (5 Marks)

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1. Primary infection: Dengue virus enters via mosquito bite → replicates in dendritic cells/macrophages → viremia → Dengue Fever (classical)
2. Secondary heterotypic infection (different serotype): Prior antibodies (non-neutralizing) → Antibody-Dependent Enhancement (ADE) → antibodies from first infection facilitate entry into more macrophages → massive viral replication
3. Immune activation: Activated T-cells and macrophages release TNF-α, IL-6, IL-8 → endothelial damage and vascular leak
4. Thrombocytopenia mechanisms:

• Bone marrow suppression by virus
• Immune-mediated platelet destruction (molecular mimicry)
• Platelet consumption (DIC)

1. Plasma leakage: Due to cytokine-mediated endothelial dysfunction → pleural effusion, ascites, hemoconcentration
2. Hemorrhage: Low platelets + coagulopathy → petechiae, purpura, gum bleeding, internal hemorrhage

• Dengue without warning signs / With warning signs / Severe Dengue (shock, organ impairment)

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Q4. Dog bite — Prophylactic Measures (5 Marks)

• Wash wound vigorously with soap and water for ≥15 minutes — mechanical removal of virus
• Apply antiseptic: 70% alcohol, povidone-iodine, or tincture iodine
• Do NOT suture the wound immediately (promotes viral inoculation into deeper tissues)
• Tetanus prophylaxis as appropriate

• Intramuscular (Essen regimen): 1 ml IM in deltoid on Days 0, 3, 7, 14, 28 (5 doses)
• Intradermal (2-site ID regimen): 0.1 ml intradermally at 2 sites on Days 0, 3, 7, 28 (WHO-approved; cost-effective)
• Vaccines: HDCV, PCECV, PVRV (Purified Vero Cell Rabies Vaccine)
• If previously vaccinated: 2 doses only (Days 0 and 3) — No RIG needed

• Human RIG (HRIG): 20 IU/kg — preferred
• Equine RIG (ERIG): 40 IU/kg — after skin test
• Infiltrate as much as possible into and around the wound
• Remaining volume: IM at anatomically distant site
• Given on Day 0 only — do NOT repeat
• Never give in same syringe or site as vaccine

• If the biting dog/cat is healthy and available for observation for 10 days → PEP can be deferred and stopped if animal remains healthy
• If animal dies, runs away, or is wild → complete full PEP

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SECTION IV — SHORT NOTES (5×5 = 25 Marks)

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1. Antibiotic Stewardship (5 Marks)

• Right antibiotic, right dose, right route, right duration
• Reduce antibiotic resistance (AMR)
• Reduce Clostridioides difficile infections, side effects, costs

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2. Opportunistic Fungal Infections + Aspergillosis in detail (5 Marks)

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• Hyaline septate mold; spores (conidia) ubiquitous in environment — inhaled from air, soil, decaying vegetation

1. Inhalation of conidia → germinate in lungs → hyphae invade blood vessels (angioinvasion)
2. Thrombosis → hemorrhagic pulmonary infarcts
3. In severely neutropenic host: no phagocytic defense → rapid dissemination to brain, kidneys, sinuses

• Voriconazole — drug of choice for invasive aspergillosis
• Isavuconazole, posaconazole — alternatives
• Amphotericin B (conventional or liposomal) — second line
• Surgical resection of aspergilloma if large/symptomatic

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3. Principles of Ethics (5 Marks)

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• Patient has the right to make decisions about their own health care
• Requires: informed consent, capacity to decide, freedom from coercion
• Includes right to refuse treatment
• Applies to: consent before procedures, disclosure of diagnosis

• The doctor must act in the best interest of the patient
• Provide treatments that are beneficial; use evidence-based medicine
• Goes beyond "do no harm" — actively promotes patient welfare

• Avoid causing unnecessary harm
• Weighing risks vs. benefits of any intervention
• Avoid futile treatment
• Applies to: avoiding unnecessary surgery, over-investigation, toxic drugs

• Fair allocation of healthcare resources
• Treat similar cases similarly (distributive justice)
• Equal access regardless of race, gender, economic status
• Relevant in: organ allocation, vaccination prioritization, clinical trials

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• Confidentiality: Keep patient information private (with defined exceptions)
• Veracity (Truth-telling): Be honest with patients about diagnosis and prognosis
• Fidelity: Keep promises made to patients; professional duty
• Sanctity of life: Respect life as inherently valuable

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4. Algorithms in HIV Diagnosis (5 Marks)

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• Detects antibodies to specific HIV proteins (gp120, gp41, gp160, p24, p31, p51, p66)
• Positive: ≥2 bands from: p24, gp41, gp120/160
• Indeterminate: 1 band present — repeat in 4–6 weeks

• Maternal IgG crosses placenta → ELISA unreliable up to 18 months
• Use HIV DNA PCR at 6 weeks, 6 months, 18 months

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5. MRSA — Resistance Mechanism and Clinical Relevance (5 Marks)

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1. Normal mechanism of beta-lactam action: Beta-lactams (penicillins, cephalosporins, carbapenems) bind to Penicillin Binding Proteins (PBPs) — transpeptidases that catalyze peptidoglycan cross-linking → inhibit cell wall synthesis → bactericidal
2. MRSA resistance via mecA gene:

• MRSA acquires the mecA gene (located on mobile genetic element — SCCmec (Staphylococcal Cassette Chromosome mec))
• mecA encodes a novel PBP called PBP2a (PBP2')
• PBP2a has extremely low binding affinity for ALL beta-lactam antibiotics
• Even in the presence of methicillin/oxacillin/all penicillins/cephalosporins: PBP2a continues cell wall synthesis → bacteria survive

1. Additional resistance mechanisms:

• Beta-lactamase (penicillinase): Hydrolyzes penicillin ring — encoded by blaZ gene (plasmid-mediated)
• Efflux pumps: NorA efflux pump — resistance to fluoroquinolones
• Biofilm formation: Particularly CA-MRSA and device-associated infections

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📊 MARKS SUMMARY

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Exam tips: Use tables wherever possible (examiners reward organised answers), write the diagnosis in the first line for applied questions, and always mention the gold standard diagnostic test explicitly. For all 5-mark answers, aim for 5–6 clear structured points.