MBBS 1st Year Professional Exam Answers

LONG ESSAY (1 × 10 = 10 marks)

Describe the main function of neutrophils in the immune system and outline the steps involved in their primary mission of neutralizing bacteria or fungi by phagocytosis. List the five main types of leukocytes and categorize them into granulocytes and agranulocytes/mononuclear lymphocytes. Draw a diagram of eosinophils on a stained microscopic slide and describe their role in defending against multicellular parasites or their involvement in allergic reactions.

a) Main Function of Neutrophils & Steps in Phagocytosis (4 marks)

b) Five Main Types of Leukocytes — Categorized (4 marks)

c) Eosinophil — Diagram & Role (3 marks)

        ┌──────────────────────────────────────┐
        │   EOSINOPHIL on Wright-stained smear │
        │                                      │
        │    ╔══════╗   ╔══════╗               │
        │    ║      ║───║      ║   ← Bilobed   │
        │    ║ Lobe ║   ║ Lobe ║     nucleus   │
        │    ╚══════╝   ╚══════╝   (purple)    │
        │                                      │
        │  ● ● ● ● ● ● ● ● ● ● ● ●            │
        │  ● ● ● ● ● ● ● ● ● ● ● ●  ← Large  │
        │  ● ● ● ● ● ● ● ● ● ● ● ●    brick-  │
        │                               red    │
        │                            granules  │
        └──────────────────────────────────────┘
   Size: ~12–15 µm | Granules: Large, uniform, bright red-orange

• Eosinophils attach to juvenile parasitic worms (e.g., Schistosoma) and kill them by:
  • Releasing hydrolytic enzymes from granules (modified lysosomes)
  • Releasing highly reactive oxygen species lethal to parasites
  • Secreting major basic protein (MBP) — a highly larvicidal polypeptide from granules
• Also activated in trichinosis (Trichinella/pork worm)

• Eosinophils accumulate in tissues undergoing allergic reactions (e.g., peribronchial tissue in asthma, skin in allergic reactions)
• This is driven by eosinophil chemotactic factor released by mast cells and basophils
• Eosinophils detoxify inflammation-inducing substances from mast cells/basophils and phagocytize allergen-antibody complexes, preventing excess spread of the inflammatory process
• However, excessive eosinophil infiltration can exacerbate disease (e.g., airway remodeling in asthma)

Sources: Guyton & Hall Medical Physiology; Medical Microbiology 9e; Henry's Clinical Diagnosis & Management

SHORT ESSAYS (5 × 4 = 20 marks)

Short Essay 1: Define homeostasis and describe various feedback mechanisms with appropriate examples.

"Homeostasis occurs at a price: energy. A well-regulated parameter is not in equilibrium but in a steady state — the body matches actions that lower the parameter with actions that raise it." — Medical Physiology

Feedback Mechanisms:

1. Sensor — detects the vital parameter (e.g., glucose receptors in pancreatic β-cells)
2. Comparator — compares the signal with an internal set-point
3. Error signal × Gain — produces an output proportional to the difference (e.g., insulin release)
4. Effector — opposes the disturbance, returning the parameter toward the set-point

• Arterial baroreceptors regulate blood pressure
• PTH and Ca²⁺ maintain calcium homeostasis
• Hypothalamus-pituitary-adrenal axis (cortisol regulation)

• Childbirth: Oxytocin causes uterine contractions → cervical distension → more oxytocin → stronger contractions (self-reinforcing until delivery)
• Blood clotting: Thrombin formation activates more clotting factors → amplifies clot formation
• Action potential: Na⁺ influx depolarizes the membrane → more Na⁺ channels open → more depolarization (positive feedback until threshold is exceeded)

Source: Medical Physiology (Boron & Boulpaep)

Short Essay 2: Describe primary and secondary active transport with appropriate examples.

Primary Active Transport

• Directly coupled to an energy source — specifically hydrolysis of ATP
• The ATPase enzyme both hydrolyzes ATP and acts as the carrier
• Can move solutes against steep electrochemical gradients

• Na⁺-K⁺ ATPase pump (Na⁺-K⁺ ATPase): The classic example. On the basolateral membrane of cells, it hydrolyzes ATP to pump 3 Na⁺ out of the cell and 2 K⁺ in — electrogenic (creates a net negative charge of ~−70 mV inside the cell). This maintains low intracellular Na⁺ and high intracellular K⁺.
• H⁺-ATPase (proton pump in renal tubule, gastric parietal cells)
• H⁺-K⁺ ATPase (gastric acid secretion)
• Ca²⁺-ATPase (SERCA pump; calcium reabsorption)

Secondary Active Transport

• Coupled indirectly to energy — uses the ion gradient established by primary active transport (typically the Na⁺ gradient) as its energy source
• Classified as:
  • Cotransport (symport): solute and Na⁺ move in the same direction
  • Countertransport (antiport): solute and Na⁺ move in opposite directions

• Na⁺-glucose cotransport (SGLT): Na⁺ gradient pulls glucose into the cell against its concentration gradient (renal proximal tubule, intestinal epithelium) — classic cotransport
• Na⁺-amino acid cotransport (intestinal absorption)
• Na⁺-Ca²⁺ exchanger (NCX): exchanges 3 Na⁺ in for 1 Ca²⁺ out — countertransport
• Na⁺-H⁺ exchanger (NHE): Na⁺ in, H⁺ out — countertransport (acid-base regulation)

Source: Guyton & Hall Textbook of Medical Physiology

Short Essay 3: A patient is suspected of having a clotting disorder. Outline the immediate response to vessel damage, ending with clot formation. Briefly mention the key cellular or soluble components involved in each step. — Three Main Steps of Hemostasis

Step 1: Vascular Spasm (Vasoconstriction)

• Local myogenic spasm — direct muscle response to trauma
• Autacoid factors from traumatized tissue, vascular endothelium, and platelets
• Nervous reflexes — pain impulses from the damaged area
• Thromboxane A₂ (TXA₂) — released by platelets; potent vasoconstrictor especially for small vessels

Step 2: Formation of the Platelet Plug

• Platelets (thrombocytes): disc-shaped, 1–4 µm, formed from megakaryocytes in bone marrow; normal count 150,000–450,000/µL
• Platelet activation: When vessel wall is damaged, collagen is exposed → platelets adhere to it via glycoprotein receptors
• Platelets release: ADP (recruits more platelets), TXA₂ (vasoconstriction + platelet aggregation), serotonin, and platelet-derived growth factor (PDGF) (promotes vascular repair)
• Platelet plug formation: Successive layers of platelets aggregate at the injury site, forming a hemostatic plug

Step 3: Blood Coagulation (Clot Formation)

• Triggered by either the extrinsic pathway (tissue factor/TF + Factor VII — activated by vessel injury) or the intrinsic pathway (Factor XII activation by exposed collagen)
• Both pathways converge on Factor X → Factor Xa, which combines with Factor Va + Ca²⁺ + phospholipid to form prothrombin activator

• Prothrombin activator (in the presence of Ca²⁺) cleaves prothrombin into thrombin
• This is the rate-limiting step

• Thrombin (a serine protease) cleaves fibrinogen into fibrin monomers
• Fibrin monomers polymerize into fibrin fibers
• Factor XIIIa (activated by thrombin) cross-links fibrin → stable clot
• The fibrin mesh enmeshes platelets, RBCs, and plasma to form the final clot

Source: Guyton & Hall Textbook of Medical Physiology, Chapter 37

Short Essay 4: A patient requires a blood transfusion. Explain the relevance of blood typing, particularly the ABO and Rh systems. What immunological reaction can occur if donor and recipient blood types are incompatible?

Blood Typing — ABO System

• All individuals produce a core glycan → fucosyltransferase adds fucose → forms the H antigen
• A gene on chromosome 9 encodes a modifying glycosyltransferase with 3 allelic variants:
  • A allele: adds N-acetylgalactosamine to H antigen → A antigen
  • B allele: adds galactose to H antigen → B antigen
  • O allele: no enzymatic activity → only H antigen expressed

Rh System

• The Rh antigen (D antigen) is a protein on RBC membranes
• Rh positive (Rh⁺): D antigen present (~85% of population)
• Rh negative (Rh⁻): D antigen absent
• Unlike ABO, Rh antibodies are not naturally present — they are formed only after exposure (transfusion or pregnancy)
• Clinically important in Hemolytic Disease of the Newborn (HDN/erythroblastosis fetalis): Rh⁻ mother sensitized by Rh⁺ fetus → anti-D IgG crosses placenta in subsequent pregnancy → destroys fetal RBCs

Immunological Reaction in ABO-Incompatible Transfusion

1. Preexisting IgM antibodies (e.g., anti-A in a type B recipient) bind to A antigens on transfused RBCs
2. Complement activation → intravascular hemolysis (RBC lysis within blood vessels)
3. Phagocytosis of antibody- and complement-coated (opsonized) erythrocytes by macrophages in the liver and spleen (extravascular hemolysis)
4. Hemoglobin released from lysed RBCs → acute renal tubular necrosis and kidney failure (hemoglobin is toxic to renal tubular cells)
5. Cytokine storm: massive release of TNF-α, IL-1 → high fever, hypotension, shock
6. Disseminated Intravascular Coagulation (DIC): widespread clotting consumes clotting factors faster than they are synthesized → paradoxical bleeding (fatal if untreated)
7. Jaundice and anemia from RBC destruction

Sources: Cellular and Molecular Immunology (Abbas); Guyton & Hall Medical Physiology