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MBBS 1st Year Professional Exam — Model Answers

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LONG ESSAY (10 Marks)

Q1. Neutrophils in the Immune System & Steps of Phagocytosis (4 marks)

Introduction

Main Functions of Neutrophils

1. Phagocytosis — engulf and destroy bacteria and fungi
2. Degranulation — release antimicrobial enzymes (elastase, myeloperoxidase, defensins) from primary (azurophilic) and secondary (specific) granules
3. Respiratory burst — generate reactive oxygen intermediates (ROIs) via NADPH oxidase to kill ingested pathogens
4. NET formation — release Neutrophil Extracellular Traps (webs of DNA + antimicrobial proteins) to trap pathogens extracellularly
5. Chemokine secretion — amplify and coordinate the inflammatory response

Steps Involved in Phagocytosis of Bacteria/Fungi

• Fc receptors — bind IgG antibodies coating the bacteria
• Complement receptors (CR3) — bind C3b opsonin on the bacterial surface
• Toll-like receptors (TLRs) — directly recognize pathogen-associated molecular patterns (PAMPs)

• Oxygen-dependent mechanisms:
  • NADPH oxidase / phox system → generates superoxide anions (O₂⁻) → converted to H₂O₂ and hydroxyl radicals
  • Myeloperoxidase (MPO) system → converts H₂O₂ + Cl⁻ → hypochlorous acid (HOCl, ~1000× more bactericidal than H₂O₂)
• Oxygen-independent mechanisms:
  • Lysozyme (digests bacterial cell wall)
  • Defensins (membrane-disrupting peptides)
  • Lactoferrin (iron chelator, bacteriostatic)
  • Elastase and cathepsin G (proteolytic enzymes)

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Q1(a). Five Main Types of Leukocytes — Granulocytes vs Agranulocytes (3 marks)

Memory aid: "Never Let Monkeys Eat Bananas" → Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils

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Q1(b). Diagram of Eosinophil & Role Against Parasites / Allergic Reactions (3 marks)

Diagram of Eosinophil on Stained Microscopic Slide

         EOSINOPHIL ON H&E STAIN

        ╔═══════════════════════════╗
        ║  ●●●●●●●●●●●●●●●●●●●●●  ║  ← Large bright pink/red
        ║  ●●●●●●●●●●●●●●●●●●●●●  ║    (eosinophilic) granules
        ║  ●● ┌─────┐ ●●●● ┌────┐ ●║
        ║  ●● │  N  │ ●●●● │ N  │ ●║  ← Bi-lobed nucleus
        ║  ●● │lobe1│ ●●●● │lob2│ ●║    (connected by thin strand)
        ║  ●● └──┬──┘ ●●●● └─┬──┘ ●║
        ║  ●●●●●│●●●●●●●●●●│●●●●●●║
        ║  ●●●●●╰────────────╯●●●●●║  ← Thin chromatin bridge
        ╚═══════════════════════════╝
        
    Key: N = nucleus lobe, ● = eosinophilic (pink) granules
    Size: ~12–17 µm diameter

• Nucleus: Characteristically bi-lobed (two lobes connected by a thin chromatin bridge)
• Cytoplasm: Filled with large, coarse, bright pink-orange (eosinophilic) granules on H&E stain
• Granule contents: Major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), eosinophil peroxidase
• Size: 12–17 µm

Role Against Multicellular Parasites

1. ADCC (Antibody-Dependent Cellular Cytotoxicity): IgE and IgG antibodies on the parasite surface bind to eosinophil Fc receptors → activates degranulation
2. Degranulation releases:
   • Major Basic Protein (MBP) — toxic to larval surface membranes
   • Eosinophil Cationic Protein (ECP) — ribonuclease activity, disrupts parasite tegument
   • Eosinophil Peroxidase — generates reactive oxygen species → oxidative killing
3. Signal of infection: peripheral blood eosinophilia is a hallmark of helminth infestation

Role in Allergic Reactions

1. Mast cells and basophils degranulate → release histamine, leukotrienes, and eosinophil chemotactic factor of anaphylaxis (ECF-A)
2. ECF-A recruits eosinophils to the site (airways in asthma, nasal mucosa in rhinitis, skin in eczema)
3. Eosinophils release MBP and ECP → amplify the inflammatory response → epithelial damage
4. They also produce Platelet Activating Factor (PAF) and Leukotriene C4 (LTC4) → bronchoconstriction

Eosinophilia (>0.5 × 10⁹/L) is the hallmark of both allergic diseases and helminthic infections.

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SHORT ESSAYS (4 marks each)

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Short Essay 1. Define Homeostasis & Describe Feedback Mechanisms

Definition

Components of a Homeostatic Control System

1. Stimulus — deviation of the variable from the set point
2. Receptor (sensor) — detects the deviation and sends information to the control centre
3. Control centre (integrating centre) — compares input with the set point and initiates an appropriate response
4. Effector — produces the corrective response

Types of Feedback Mechanisms

Example: Thermoregulation

• Body temp rises → detected by hypothalamic thermoreceptors → effectors (sweat glands, vasodilation) activated → heat loss → temperature returns to 37°C

Example: Blood glucose regulation

• Blood glucose rises → pancreatic β-cells secrete insulin → glucose uptake by cells → blood glucose falls back to normal

Example: Parturition (childbirth)

• Uterine stretching → oxytocin release → stronger contractions → more stretching → more oxytocin → cycle ends at delivery

Example: Blood clotting (coagulation cascade)

• Platelet adhesion → ADP release → recruits more platelets → plug grows → culminates in clot formation

Example: Action potential (all-or-nothing)

• Na⁺ influx → depolarization → more Na⁺ channels open → more influx → completed in milliseconds

Example: Increase in heart rate and breathing rate before exercise begins (conditioned anticipatory response)

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Short Essay 2. Primary & Secondary Active Transport with Examples

Active Transport — Definition

Primary Active Transport

• Transport protein (ATPase pump) binds ATP → undergoes conformational change → translocates ion across the membrane against its gradient → ATP is hydrolyzed to ADP + Pᵢ

Most important: Na⁺/K⁺-ATPase — uses ~30% of all cellular ATP; critical for excitability of neurons and muscle cells.

Secondary Active Transport

• Symport (co-transport): Both substances move in the same direction
• Antiport (counter-transport/exchange): Substances move in opposite directions

Summary Comparison

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Short Essay 3. Three Main Steps of Hemostasis (Clotting Disorder)

Step 1 — Vascular Spasm (Primary Response)

• Immediate response (seconds) to vessel injury
• Smooth muscle of the vessel wall contracts → vasoconstriction → reduces blood flow through the damaged vessel
• Triggered by: direct smooth muscle damage, nervous reflexes, local chemical mediators (endothelin, thromboxane A₂)
• Components: vascular smooth muscle

Step 2 — Primary Hemostasis — Platelet Plug Formation

• Occurs over 1–3 minutes
• Platelet Adhesion: Exposed subendothelial collagen and von Willebrand Factor (vWF) bind platelet receptors (GP Ib-IX-V for vWF; GP Ia/IIa and GP VI for collagen)
• Platelet Activation: Binding activates platelets → shape change (disc → spiky sphere) → release of granule contents:
  • Dense granules: ADP, serotonin, Ca²⁺
  • Alpha granules: fibrinogen, vWF, platelet factor 4, P-selectin
• ADP and TXA₂ (thromboxane A₂) recruit additional platelets
• Platelet Aggregation: GP IIb/IIIa receptors on activated platelets bind fibrinogen (and vWF) as bridges between adjacent platelets → formation of the primary platelet plug (loose, temporary)
• Components: platelets, vWF, fibrinogen, collagen

Step 3 — Secondary Hemostasis — Coagulation Cascade → Clot Formation

• Stabilises the platelet plug with a fibrin mesh (over 3–6 minutes)
• The coagulation cascade is triggered by:
  • Extrinsic pathway: Tissue factor (TF / thromboplastin) released from damaged cells + Factor VII → TF-VIIa complex → activates Factor X
  • Intrinsic pathway: Exposed collagen activates Factor XII → cascade through XI → IX → VIII → X
  • Common pathway: Factor Xa + Va (prothrombinase complex) → converts prothrombin → thrombin → converts fibrinogen → fibrin monomers → fibrin polymers cross-linked by Factor XIII → stable clot
• Thrombin is the key enzyme: activates more clotting factors (positive feedback), activates platelets, and activates fibrinogen

Clinical note: Defects in Step 2 (e.g., von Willebrand disease, thrombocytopenia) prolong the bleeding time. Defects in Step 3 (e.g., haemophilia A/B — Factor VIII/IX deficiency) prolong the aPTT/PT.

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Short Essay 4. Blood Typing — ABO & Rh Systems in Transfusion

Relevance of Blood Typing in Transfusion

ABO Blood Group System

Rh (Rhesus) Blood Group System

• Based on the D antigen (most immunogenic): individuals are Rh-positive (Rh⁺) if D antigen is present, Rh-negative (Rh⁻) if absent
• ~85% of the population is Rh⁺
• Unlike ABO: Rh antibodies are immune (acquired) — only produced after exposure to Rh-positive blood (via transfusion or pregnancy)

Immunological Reaction with Incompatible Blood

1. Recipient has pre-formed antibodies against donor RBC antigens (e.g., type A recipient receiving type B blood → anti-B IgM attacks donor B cells)
2. Complement activation (classical pathway): Antigen-antibody complexes activate C1 → C3 → C5 → Membrane Attack Complex (MAC)
3. Intravascular haemolysis: MAC punches holes in RBC membranes → immediate haemolysis → release of haemoglobin → haemoglobinaemia, haemoglobinuria
4. Cytokine storm: Anaphylatoxins C3a and C5a → mast cell degranulation → histamine → hypotension, bronchospasm
5. DIC (disseminated intravascular coagulation) may ensue

• Fever, chills, back/flank pain, hypotension
• Haemoglobinuria (red/cola-coloured urine)
• Acute kidney injury (haemoglobin tubular toxicity)
• DIC, shock — can be fatal

• Rh⁻ mother carrying Rh⁺ fetus → at delivery, fetal RBCs enter maternal circulation → mother forms anti-D IgG
• In subsequent Rh⁺ pregnancies: maternal anti-D IgG crosses the placenta → destroys fetal RBCs → erythroblastosis fetalis (hydrops fetalis in severe cases)
• Prevented by anti-D immunoglobulin (Rhogam) given to Rh⁻ mothers at 28 weeks and within 72 hours of delivery

Prevention

• Pre-transfusion testing: ABO grouping, Rh typing, antibody screening, cross-matching
• Universal donor for emergency: O-negative packed RBCs
• Universal plasma donor: AB plasma

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Summary of Mark Allocation

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