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Here are the answers to all 20 questions from this Tamil Nadu Dr. M.G.R. Medical University MBBS Physiology MCQ paper:

Answer Key - Physiology Paper I (MBBS 0726, July 2026, Set D)

Q#	Question (Short)	Answer	Explanation
1	Insulin receptor type	C) Tyrosine kinase receptor	Insulin receptor is a receptor tyrosine kinase (RTK) - it has intrinsic enzymatic activity and phosphorylates tyrosine residues on activation.
2	Hormone for male external genitalia development in fetal life	C) Dihydrotestosterone (DHT)	DHT (formed by 5-alpha reductase action on testosterone) is responsible for development of the male external genitalia and prostate during fetal life. Testosterone itself drives internal duct development.
3	Most common site of fertilization	D) Ampulla of the fallopian tube	Fertilization most commonly occurs in the ampulla of the fallopian tube (uterine tube), specifically the ampullary-isthmic junction region.
4	Major intracellular anion	B) Potassium	Wait - Potassium is the major intracellular cation. The major intracellular anion is phosphate (HPO4²-/proteins). However, among the given options, B) Potassium is incorrect as it is a cation. The closest correct answer among the choices is that none perfectly fit, but if the question is about the major intracellular ion overall, Potassium is the answer intended (the question may have an error - potassium is the major intracellular cation, not anion). Among the options given, B) Potassium is the expected answer.
5	GLUT transport type	B) Facilitated diffusion	GLUT (glucose transporter) proteins mediate facilitated diffusion - they move glucose down its concentration gradient without energy expenditure.
6	Equilibrium potential for an electrolyte	A) Nernst Equation	The Nernst equation calculates the equilibrium (reversal) potential for a single ion species. The Goldman equation accounts for multiple ions.
7	Release of ADP + Pi from myosin head is responsible for	C) Power stroke	During cross-bridge cycling: ATP binds → myosin detaches from actin → ATP hydrolysis (→ADP+Pi) cocks the head → myosin binds actin → release of ADP+Pi drives the power stroke (rowing motion).
8	Vitamin K-dependent coagulation factor	B) Prothrombin	Prothrombin (Factor II) is Vitamin K-dependent. Others: Fibrinogen (Factor I) is NOT Vit K-dependent. Labile factor (V) is NOT Vit K-dependent. Hageman factor (XII) is NOT Vit K-dependent. The Vit K-dependent factors are II, VII, IX, X, Protein C, Protein S.
9	Myosin and actin filaments are kept in place by	D) Titin	Titin (connectin) is the giant elastic protein that anchors myosin thick filaments to the Z-disc and maintains sarcomere structure. Actinin anchors actin to Z-discs.
10	Erythropoietin is mainly produced by	C) Kidney	In adults, ~90% of EPO is produced by peritubular interstitial cells of the kidney (specifically cortical interstitial fibroblasts). The liver produces a small amount.
11	Plasma protein responsible for colloid osmotic pressure	C) Albumin	Albumin accounts for ~80% of plasma colloid osmotic pressure (oncotic pressure) due to its high concentration and the Gibbs-Donnan effect.
12	Extrinsic pathway of coagulation is initiated by	B) Tissue Factor	The extrinsic (tissue factor) pathway is initiated when tissue factor (thromboplastin/Factor III) is exposed and binds Factor VIIa.
13	Hormone for gallbladder contraction and pancreatic enzyme secretion	B) Cholecystokinin (CCK)	CCK is released from I-cells of the duodenum/jejunum and stimulates both gallbladder contraction and pancreatic enzyme secretion. Secretin stimulates bicarbonate secretion.
14	Slow waves in GI smooth muscle are generated by	C) Interstitial cells of Cajal (ICC)	ICC are the pacemaker cells of the GI tract, located in the myenteric plexus region, generating the basic electrical rhythm (slow waves).
15	Brunner's glands are primarily found in	C) Duodenum	Brunner's (duodenal) glands are submucosal glands found in the duodenum; they secrete alkaline mucus to neutralize gastric acid.
16	Not reabsorbed in proximal convoluted tubule	D) Creatinine	Creatinine is filtered at the glomerulus and is neither reabsorbed nor significantly metabolized; it is actually slightly secreted. Glucose, amino acids, and sodium are all actively reabsorbed in the PCT.
17	Hormone that increases water reabsorption in collecting duct	C) Vasopressin (ADH)	Vasopressin (antidiuretic hormone, ADH) acts on V2 receptors in the collecting duct to insert aquaporin-2 channels, increasing water reabsorption. This is deficient/resistant in Diabetes Insipidus.
18	Main role of kidneys in acid-base balance	B) Excreting hydrogen ions and reabsorbing bicarbonate	The kidneys regulate acid-base by excreting H⁺ (via titratable acidity and ammonium) and reabsorbing/regenerating HCO₃⁻. In DKA (metabolic acidosis), this compensatory mechanism is engaged.
19	Diagnosis: trauma, femur fracture, feeble pulse, HR 120, BP 80/60	A) Hypovolemic shock	Blood loss from femur fracture causes hypovolemic (hemorrhagic) shock: tachycardia, hypotension, feeble pulse. Distributive shock (septic/anaphylactic) has different etiologies.
20	Gland that secretes Melatonin	B) Pineal gland	Melatonin is synthesized and secreted by the pineal gland (from tryptophan → serotonin → melatonin), regulated by light-dark cycles via the suprachiasmatic nucleus.

Quick Summary of Correct Answers: C, C, D, B, B, A, C, B, D, C, C, B, B, C, C, D, C, B, A, B

Note on Q4: The question asks for the "major intracellular anion" but lists Potassium (a cation) as an option. Potassium is the major intracellular cation; the major intracellular anions are phosphate and proteins. This appears to be a question error - the intended answer is likely B) Potassium as the "major intracellular ion" (cation), which is a classic exam fact.This is Set A of the same Physiology paper. I can see the student has already marked some answers (tick marks visible). Let me provide the correct answers and flag where the student went wrong.

Answer Key - Set A (MBBS 0726, July 2026, Physiology Paper I)

Q#	Question (Short)	Correct Answer	Student Marked	Status
1	Major intracellular anion	B) Potassium (see note below)	B) Potassium ✓	✅
2	GLUT transport type	B) Facilitated diffusion	B) Facilitated diffusion ✓	✅
3	Equilibrium potential equation	A) Nernst Equation	B) Donnan Gibbs Equation ✗	❌
4	Release of ADP+Pi from myosin head → responsible for	C) Power stroke	C) Power stroke ✓	✅
5	Vitamin K-dependent coagulation factor	B) Prothrombin	B) Prothrombin ✓	✅
6	Myosin and actin kept in place by	D) Titin	D) Titin ✓	✅
7	EPO mainly produced by	C) Kidney	C) Kidney ✓	✅
8	Plasma protein for colloid osmotic pressure	C) Albumin	C) Albumin ✓	✅
9	Extrinsic pathway initiated by	B) Tissue Factor	B) Tissue Factor ✓	✅
10	Gallbladder contraction + pancreatic enzyme secretion	B) Cholecystokinin	B) Cholecystokinin ✓	✅
11	GI slow waves generated by	C) Interstitial cells of Cajal	A) Sympathetic nerves ✗	❌
12	Brunner's glands found in	C) Duodenum	C) Duodenum ✓	✅
13	Not reabsorbed in PCT	D) Creatinine	D) Creatinine ✓	✅
14	Hormone increases water reabsorption in collecting duct	C) Vasopressin	A) Aldosterone ✗	❌
15	Kidneys' main role in acid-base balance	B) Excreting H⁺ and reabsorbing HCO₃⁻	C) Excreting HCO₃⁻ and reabsorbing H⁺ ✗	❌
16	Femur fracture + BP 80/60 + HR 120 = diagnosis	A) Hypovolemic shock	A) Hypovolemic shock ✓	✅
17	Gland secreting Melatonin	B) Pineal gland	B) Pineal gland ✓	✅
18	Insulin receptor type	C) Tyrosine kinase receptor	D) Nuclear receptor ✗	❌
19	Hormone for male external genitalia in fetal life	C) Dihydrotestosterone	C) Dihydrotestosterone ✓	✅
20	Most common site of fertilization	D) Ampulla of the fallopian tube	D) Ampulla of the fallopian tube ✓	✅

Score: ~15/20 (estimated from visible marks)

Corrections - Where the Student Went Wrong

❌ Q3 - Equilibrium Potential Equation

Student answered: Donnan Gibbs Equation
Correct answer: Nernst Equation
Why: The Nernst equation calculates the equilibrium (reversal) potential for a single ion at a given concentration gradient: E = (RT/zF) × ln([ion outside]/[ion inside]). The Donnan-Gibbs equation deals with membrane equilibrium across a semipermeable membrane with non-diffusible ions (relevant to plasma oncotic pressure). The Goldman equation calculates membrane potential when multiple ions are involved.

❌ Q11 - GI Slow Waves

Student answered: Sympathetic nerves
Correct answer: Interstitial cells of Cajal (ICC)
Why: ICC are the pacemaker cells of the GI tract, located in the myenteric plexus between the circular and longitudinal muscle layers. They generate the basic electrical rhythm (slow waves / pacesetter potentials). Sympathetic/parasympathetic nerves modulate the amplitude of contractions but do NOT generate slow waves.

❌ Q14 - Water Reabsorption in Collecting Duct

Student answered: Aldosterone
Correct answer: C) Vasopressin (ADH)
Why: Vasopressin acts on V2 receptors in the collecting duct → activates adenylyl cyclase → cAMP → inserts aquaporin-2 channels → increases water reabsorption. Aldosterone acts on the collecting duct to reabsorb sodium (not water directly). Diabetes Insipidus is specifically a vasopressin deficiency/resistance disorder.

❌ Q15 - Kidney's Role in Acid-Base Balance

Student answered: Excreting bicarbonate and reabsorbing hydrogen ions
Correct answer: B) Excreting hydrogen ions and reabsorbing bicarbonate
Why: The kidney maintains acid-base by: (1) reabsorbing filtered HCO₃⁻ (mainly in PCT), and (2) excreting H⁺ as titratable acid and ammonium (NH₄⁺) in the collecting duct. In DKA (metabolic acidosis), the kidney compensates by maximally excreting H⁺ and regenerating HCO₃⁻. The option chosen was the exact reverse - a common reversal error.

❌ Q18 - Insulin Receptor Type

Student answered: Nuclear receptor
Correct answer: C) Tyrosine kinase receptor
Why: Nuclear receptors are for lipid-soluble hormones (steroids, thyroid hormone, Vit D) that enter the cell and act on DNA directly. Insulin is a large peptide hormone - it cannot enter the cell. It binds its receptor on the cell surface, which has intrinsic tyrosine kinase activity in its intracellular domain. On binding, it autophosphorylates tyrosine residues and initiates the signaling cascade (IRS-1/2 → PI3K → Akt → GLUT4 translocation).

Note on Q1: "Major intracellular anion" with Potassium as the answer is a question error - Potassium is a cation. The actual major intracellular anion is phosphate/proteins. This question likely intended to ask about the major intracellular ion or cation, and Potassium is the standard exam answer for that.

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