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Chirayu University - Physiology-I MCQ Answer Key

Section A - Q1 (All 2 marks each)

a. Dynein protein is involved in:

Answer: ii. Endocytosis

Dynein is a minus-end-directed microtubule motor protein. It is involved in retrograde axonal transport (moving cargo from axon terminals back to the cell body) and critically in endocytosis - it pulls endosomal vesicles along microtubules toward the cell center (minus end). Dynein is distinct from active transport (which broadly refers to membrane pumps), exocytosis (which uses kinesin and actin), and apoptosis (not a transport motor function).

Ganong's Review of Medical Physiology confirms dynein as a molecular motor for axonal transport along microtubules.

b. Babu Rao - 65-year-old with post-MI heart failure, decreased ejection fraction:

Answer: iii. End systolic volume must be increased

In systolic heart failure (reduced ejection fraction after MI):

The ventricle contracts poorly, so it cannot eject as much blood as normal.
This means more blood remains in the ventricle after systole → End Systolic Volume (ESV) is increased.
Ejection fraction = Stroke Volume / End Diastolic Volume. A low EF with elevated EDV means ESV is disproportionately high.
Stroke volume is actually decreased (not increased) in systolic HF.
EDV may be normal or increased (due to compensatory dilation).

Harrison's and Fuster and Hurst's The Heart confirm that in HFrEF, end-systolic volume rises as systolic function declines.

c. Carbon monoxide poisoning is an example of which type of hypoxia?

Answer: ii. Histotoxic

Wait - let me reconsider this carefully:

CO binds hemoglobin with ~250x greater affinity than O₂, forming carboxyhemoglobin. This reduces the oxygen-carrying capacity of blood - which is actually anemic hypoxia (also called hemic hypoxia). However, CO also inhibits cytochrome c oxidase (Complex IV of the electron transport chain) at the tissue level, which is the mechanism of histotoxic hypoxia.

The standard physiology teaching (Ganong, Guyton) classifies CO poisoning as Anemic (Hemic) hypoxia at the hemoglobin level, but Plum and Posner states: "Histotoxic hypoxia results from agents that poison the electron transport chain. Such agents include cyanide and carbon monoxide."

For MBBS First Year Physiology exams, CO poisoning is most commonly classified as Histotoxic hypoxia because CO poisons mitochondrial cytochrome oxidase, preventing cells from utilizing O₂ - this is the classic exam answer.

Answer: ii. Histotoxic

(Note: Some texts also classify it as anemic/hemic since CO reduces O₂ carrying capacity. The histotoxic answer is the standard for first-year physiology exams.)

d. Fatima Ali - 35-year-old female, iron deficiency anemia:

Answer: iv. Low MCHC

Iron deficiency anemia is a microcytic, hypochromic anemia:

MCV: Low (microcytic)
MCH: Low (less Hb per cell)
MCHC: Low (hypochromic - reduced hemoglobin concentration per cell)

The characteristic finding is low MCHC, which is the hallmark of iron deficiency. The textbook confirms: "A decrease in MCHC is typically used as an indicator for iron deficiency." - Tietz Textbook of Laboratory Medicine

High MCV and High MCH are seen in megaloblastic (B12/folate deficiency) anemia, not iron deficiency.

e. Which structure is involved in smooth muscle contraction?

Answer: ii. Calmodulin

In smooth muscle, there is no troponin (troponin is exclusive to cardiac and skeletal muscle). Smooth muscle uses a calmodulin-based regulatory system:

Ca²⁺ enters the cell → binds calmodulin → Ca²⁺-calmodulin complex activates myosin light chain kinase (MLCK) → MLCK phosphorylates myosin → cross-bridge cycling and contraction.
Troponin (i): skeletal and cardiac muscle only
Tropomyosin (iii): present in smooth muscle but the KEY regulatory molecule is calmodulin
Kinesin (iv): a microtubule motor protein, not involved in muscle contraction

Harper's Biochemistry and Histology (Eroschenko) confirm: calmodulin is the key regulatory protein in smooth muscle contraction.

f. Vishal - 45-year-old, chest pain, ECG question - which is NOT true?

Answer: ii. V6 is placed on the 2nd intercostal space

ECG lead placement:

Lead II: positive end at left leg, negative end at right arm ✓ (option i is TRUE)
V6: placed at the mid-axillary line, 5th intercostal space - NOT 2nd intercostal space (option ii is FALSE = NOT true)
Lead III: negative end at left arm, positive end at left leg; iii says "negative end at left wrist" - this is also debatable terminology.
V5: 5th intercostal space, anterior axillary line ✓

Standard precordial placement:

V1: 4th ICS, right sternal border
V2: 4th ICS, left sternal border
V3: between V2 and V4
V4: 5th ICS, midclavicular line
V5: 5th ICS, anterior axillary line
V6: 5th ICS, mid-axillary line

V6 is NOT on the 2nd intercostal space - option ii is the false statement.

g. All of the following are stimulations for secretion of gastrin EXCEPT:

Answer: ii. Secretin

Stimulants of gastrin secretion:

Protein digestion products (peptides, amino acids) ✓
Histamine ✓ (via H₂ receptors on parietal cells; also stimulates gastrin indirectly)
Acetylcholine (vagal stimulation) ✓
Gastric distension
Alcohol, caffeine

Secretin is released from S-cells of the duodenum in response to acid and inhibits gastrin secretion - it is an antagonist of gastric acid secretion (the "hand brake" to gastrin). Mulholland and Greenfield's Surgery confirms secretin produces paradoxical gastrin release only in gastrinoma patients; normally it inhibits gastrin.

h. 10-year-old boy, swelling/purple patches on knee and thigh, appear after minor trauma, normal bleeding time, increased clotting time, normal TLC, decreased hemoglobin:

Answer: iii. His clotting factor VIII is deficient

Analysis of the clues:

Appears relatedly after minor trauma → hemarthrosis / deep tissue bleeds pattern
Normal bleeding time: rules out platelet problems (von Willebrand mild, thrombocytopenia)
Increased clotting time (PTT/APTT prolonged): problem in the intrinsic coagulation pathway
Normal TLC: no infection/inflammation
Decreased Hb: from chronic blood loss

This is classic Hemophilia A - Factor VIII deficiency:

Hemophilia A is X-linked recessive (affects males)
Bleeding time is normal (platelet plug forms normally)
APTT/clotting time is prolonged (intrinsic pathway defect)
Purple patches = hemarthroses and muscle hematomas

Option iii is correct. The boy does NOT have megaloblastic anemia (no macrocytosis clue) and does NOT have ITP (normal TLC, normal bleeding time, and ITP has reduced platelets).

i. All of the following are examples of primary active transport EXCEPT:

Answer: iii. Na⁺-glucose cotransport

Primary active transport uses ATP directly to move substances against their gradient:

H⁺-K⁺-ATPase pump ✓ (gastric parietal cells)
Na⁺-K⁺-ATPase ✓ (ubiquitous)
SERCA (Sarcoplasmic/Endoplasmic Reticulum Ca²⁺-ATPase) ✓

Na⁺-glucose cotransport is secondary active transport - it uses the electrochemical gradient of Na⁺ (established by the Na⁺-K⁺-ATPase) to drive glucose uptake. No ATP is consumed directly. This is the SGLT transporter system (e.g., in renal tubule and intestinal epithelium).

Quick Summary Table

Q	Answer	Key Concept
a	ii. Endocytosis	Dynein = minus-end microtubule motor
b	iii. ESV must be increased	Systolic HF → poor ejection → ESV ↑
c	ii. Histotoxic	CO inhibits cytochrome oxidase
d	iv. Low MCHC	Iron deficiency = microcytic hypochromic
e	ii. Calmodulin	No troponin in smooth muscle
f	ii. V6 on 2nd ICS	V6 is at 5th ICS, mid-axillary line
g	ii. Secretin	Secretin inhibits gastrin secretion
h	iii. Factor VIII deficient	Hemophilia A: ↑ clotting time, normal BT
i	iii. Na⁺-glucose cotransport	Secondary, not primary active transport

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Here are the answers for what's visible in this image:

j. All of the following are physiological choleretics EXCEPT:

Answer: iv. Gastrin

A choleretic is any substance that increases bile secretion (bile flow) from the liver. The physiological choleretics are:

Bile salts (i) ✓ - The most potent physiological choleretic. Bile salts undergo enterohepatic circulation; when they return to the liver via the portal vein, they stimulate more bile secretion. This is the "bile salt-dependent" fraction of bile flow.
Primary bile acids (ii) ✓ - Cholic acid and chenodeoxycholic acid (primary bile acids) directly stimulate bile secretion from hepatocytes - same mechanism as bile salts.
Secretin (iii) ✓ - Released from S-cells of the duodenum in response to acid; it stimulates ductal cells of the bile duct (cholangiocytes) to secrete a large volume of bicarbonate-rich bile, significantly increasing bile flow.

Gastrin (iv) primarily stimulates gastric acid secretion by parietal cells. It does NOT significantly increase bile flow and is not classified as a physiological choleretic. While gastrin has weak choleretic effects at pharmacological doses, it is not considered a physiological choleretic.

Section B - Q2a: Ramlal, 65-year-old male, difficulty breathing, allergic to dust, pulmonary function tests...

This is a long essay question - the full question is partially cut off, but based on the visible text (65-year-old male, difficulty breathing, intermittently, history of being allergic to dust, pulmonary function tests), this is clearly describing bronchial asthma with obstructive lung disease. The question likely asks about PFT findings and/or mechanisms.

If the full question asks about PFT findings in asthma (obstructive pattern):

Parameter	Expected Finding	Reason
FEV₁	↓ Decreased	Airway narrowing limits expiratory flow
FVC	Normal or mildly ↓	Lung capacity relatively preserved
FEV₁/FVC ratio	↓ <0.70	Hallmark of obstruction
TLC	↑ Increased	Air trapping / hyperinflation
RV (Residual Volume)	↑ Increased	Air trapping due to bronchospasm
FRC	↑ Increased	Hyperinflation
PEFR	↓ Decreased	Reduced peak expiratory flow rate
Reversibility test (post-bronchodilator)	FEV₁ improves ≥12% and ≥200 mL	Confirms asthma (reversible obstruction)

Pathophysiology: Allergen (dust) → IgE-mediated mast cell degranulation → histamine, leukotrienes, prostaglandins → bronchospasm + mucosal edema + mucus hypersecretion → airway narrowing → obstructive pattern on spirometry.

Please share the complete Q2a and any remaining questions (Section B, C, etc.) if you'd like full long-essay answers written out!

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