Answer Key with Brief Explanation

First set

• Heat-labile toxin (LT) that increases cAMP
• Heat-stable toxin (ST) that increases cGMP

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Second set

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Third set

High-yield concepts behind the MCQs

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A. General Microbiology and Sterilization

1. Bacterial morphology: Cocci in chains

• Cocci are classified by their arrangement after cell division.
• Staphylococci divide in multiple planes, forming irregular grape-like clusters.
• Streptococci divide in one plane and remain attached, forming chains.
• Diplococci occur in pairs, for example Streptococcus pneumoniae and Neisseria.
• Tetrads occur in groups of four, classically Micrococcus.

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2. Bacterial oxygen requirement

• Can grow both in the presence and absence of oxygen.
• Grows better when oxygen is available because aerobic respiration produces more energy.

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3 and 4. Horizontal gene transfer through direct contact/sex pilus

• A donor bacterium containing an F factor or fertility plasmid attaches to a recipient cell.
• Transfer occurs through a sex pilus/conjugation bridge.
• R plasmids carrying multidrug-resistance genes commonly spread by conjugation.

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4. Streptococcus pyogenes antiphagocytic virulence factor

• M protein
• Hyaluronic acid capsule
• Streptolysin O and S
• Streptokinase
• Hyaluronidase
• C5a peptidase
• Pyrogenic exotoxins

• Major virulence factor of S. pyogenes.
• Prevents phagocytosis.
• Interferes with complement-mediated opsonization, especially C3b deposition.
• Produces type-specific immunity.
• Molecular mimicry involving M protein is important in acute rheumatic fever.

• Streptolysin O: haemolysin; antigenic, measured by ASO titre.
• Streptokinase: converts plasminogen to plasmin and promotes spread.
• Hyaluronidase: spreading factor that breaks down connective tissue.

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5. Moist heat under pressure

• 121°C
• 15 pounds/in² pressure
• 15 minutes

• Moist heat causes coagulation and denaturation of microbial proteins.
• It kills bacteria, fungi, viruses, and spores when correctly used.

• Surgical instruments
• Culture media
• Dressings
• Rubber goods
• Linen

• Hot air oven: dry heat, usually 160°C for 2 hours.
• Incineration: destroys contaminated waste, loops, infected materials.
• Tyndallization: intermittent steaming, now rarely used.

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6. Sterilization of heat-sensitive devices

• Plastic catheters
• Endotracheal tubes
• Artificial heart valves
• Electronic equipment
• Delicate instruments
• Some endoscopic equipment

• Alkylates proteins, DNA, and RNA.
• Effective against bacteria, viruses, fungi, and spores.

• Slow process
• Toxic and potentially carcinogenic
• Requires aeration after sterilization
• Explosive, so it is mixed with inert gas

• Alcohol and chlorhexidine are antiseptics/disinfectants, not reliable sporicidal sterilants.
• Sodium hypochlorite is a surface disinfectant.

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B. Disinfection and Hospital Infection Control

13. Flexible gastrointestinal endoscope

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15. Stethoscope and thermometer

• First remove visible soil by cleaning.
• Then use compatible low-level disinfection, commonly an alcohol-based wipe where appropriate.

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16. Blood spill on a hospital surface

1. Wear gloves and appropriate PPE.
2. Remove visible organic material carefully.
3. Apply an appropriate hospital-grade disinfectant or recommended sodium hypochlorite solution.
4. Allow adequate contact time.
5. Dispose of cleaning materials as biomedical waste.

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C. Enteric Fever and Vaccines

7. Enteric fever clinical diagnosis

• Prolonged fever
• Headache, malaise, abdominal discomfort
• Constipation in the early stage, sometimes diarrhoea later
• Relative bradycardia
• Hepatosplenomegaly
• Rose spots over trunk/abdomen

• Salmonella enterica serovar Typhi: typhoid fever
• Salmonella Paratyphi A, B, and C: paratyphoid fever

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17. Typhoid conjugate vaccine

• It can be given to young children.
• It produces a T-cell-dependent immune response.
• It induces immunological memory.
• It provides longer-lasting protection than unconjugated Vi polysaccharide vaccine.

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20. OPV versus IPV

• Replicates in the intestine.
• Generates local mucosal immunity.
• Reduces intestinal multiplication and faecal shedding.
• Helps interrupt person-to-person spread.

• It contains killed virus.
• It cannot revert to neurovirulence.
• It does not cause VAPP.

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D. Diarrhoeagenic E. coli and Cholera

8. ETEC with heat-labile and heat-stable toxins

• Traveller's diarrhoea
• Infantile diarrhoea in developing countries
• Food- and water-borne watery diarrhoea

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18. Bloody diarrhoea after undercooked beef

• Causes haemorrhagic colitis.
• Is associated with undercooked beef, unpasteurized milk, contaminated vegetables, etc.
• Important serotype: O157:H7.
• Produces Shiga-like toxin, also called verotoxin.

• Inactivates the 60S ribosomal subunit.
• Inhibits protein synthesis.
• Produces mucosal injury and bloody diarrhoea.

• Haemolytic uraemic syndrome: microangiopathic haemolytic anaemia, thrombocytopenia, and acute kidney injury.

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19. ETEC heat-labile toxin resembling cholera toxin

1. Toxin binds intestinal epithelial cells.
2. Activates adenylate cyclase.
3. Increases intracellular cAMP.
4. Causes chloride and water secretion into the lumen.
5. Produces secretory watery diarrhoea.

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9. Rice-water stools in cholera

• Is an A-B exotoxin.
• B subunit binds GM1 ganglioside receptors on enterocytes.
• A subunit activates adenylate cyclase.
• Intracellular cAMP rises.
• Chloride, bicarbonate, water, and electrolytes are secreted into intestinal lumen.

• Profuse painless watery diarrhoea
• Rice-water stools
• Rapid dehydration
• Hypokalaemia and metabolic acidosis

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E. Gas Gangrene

10. Traumatic wound with crepitus and foul discharge

• Traumatic contaminated wound
• Devitalized, poorly oxygenated tissue
• Severe pain and rapidly increasing swelling
• Myonecrosis
• Crepitus due to gas production
• Foul-smelling discharge
• Shock and haemolysis in severe disease

• Alpha toxin: lecithinase/phospholipase C
• Causes cell membrane destruction, haemolysis, tissue necrosis, and shock.

• Urgent surgical debridement is essential.
• Antibiotics and, in selected cases, hyperbaric oxygen may be used.

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F. Exotoxins and Staphylococcal Toxins

6. General properties of bacterial exotoxins

• Proteins secreted by Gram-positive or Gram-negative bacteria.
• Highly potent and often highly specific in action.
• Usually heat-labile, though there are exceptions.
• Antigenic.
• Neutralized by antitoxin antibodies.
• Convertible to toxoids for vaccination, for example tetanus and diphtheria toxoids.

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7. Toxic shock syndrome after tampon use

• Binds MHC class II molecules on antigen-presenting cells and T-cell receptors outside the usual antigen-binding site.
• Activates a large number of T cells nonspecifically.
• Causes massive cytokine release.

• High fever
• Hypotension
• Diffuse erythematous rash
• Multiorgan dysfunction
• Later desquamation of palms and soles

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9. Sudden vomiting after cream-filled pastries

• Food is contaminated by a food handler.
• S. aureus grows in food and produces enterotoxin before ingestion.
• The preformed toxin is heat stable.
• Reheating food may kill bacteria but may not destroy toxin.

• Very short incubation, often 1-6 hours
• Sudden severe nausea and vomiting
• Abdominal cramps
• Usually little or no fever
• Diarrhoea may occur
• Self-limited illness

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10. Vomiting after reheated fried rice

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Rapid recall list

• M protein: antiphagocytic virulence factor of S. pyogenes
• Autoclave: moist heat under pressure
• ETO: heat-sensitive equipment
• GI endoscope: high-level disinfection
• Stethoscope: low-level disinfection
• Enteric fever: prolonged fever, rose spots, relative bradycardia
• TCV: preferred typhoid vaccine for routine programmes
• ETEC: LT-cAMP, ST-cGMP, watery diarrhoea
• EHEC: bloody diarrhoea, Shiga toxin, HUS
• Cholera toxin: cAMP, rice-water stool
• Conjugation: sex pilus and plasmid transfer
• Gas gangrene: C. perfringens, alpha toxin
• TSST-1: tampon-associated toxic shock syndrome
• S. aureus: preformed toxin, vomiting after cream pastries
• B. cereus: fried rice, emetic toxin
• OPV: intestinal IgA and reduced transmission
• IPV: systemic protection and no VAPP