INI-CET 2025 — Physiology
11 Previous Year Questions with Answers & Explanations
A single muscle twitch lasts 40 milliseconds. What is the minimum tetanization frequency required to produce a sustained (fused) contraction in this muscle?
Which type of pulse is based on the Frank-Starling law?
Which of the following tissues will not be able to take up glucose in insulin resistance/insulin absence/diabetes mellitus?
Which of the following is not a mechanism of action of ADH?
Patient presents to OPD with fever. Which area is most likely involved?
Which of the following is an example of feed forward mechanism?
In a normal awake person at rest with eyes closed, EEG waves that are reduced on opening the eyes:
Which of the following are features of Bezold Jarisch reflex? 1. Bradycardia 2. Hypertension 3. Coronary vasodilation 4. Tachycardia
Surface tension of the fluid lining the alveoli increases during:
Which of the following is seen in high altitude?
INI-CET 2025 - Physiology INI-CET Practice Questions and MCQs
Question 1: A single muscle twitch lasts 40 milliseconds. What is the minimum tetanization frequency required to produce a sustained (fused) contraction in this muscle?
- A. 10 Hz
- B. 20 Hz
- C. 25 Hz (Correct Answer)
- D. 40 Hz
Explanation: ### **Explanation: Tetanization Frequency** The correct answer is **25 Hz**. #### **1. Understanding the Concept** **Tetanization** occurs when a muscle is stimulated at a high enough frequency that individual twitches fuse into a single, sustained contraction. This happens because the muscle does not have enough time to relax between successive stimuli. The formula to calculate the **critical fusion frequency (tetanization frequency)** is: $$\text{Frequency (Hz)} = \frac{1}{\text{Twitch Duration (seconds)}}$$ * **Given:** Twitch duration = 40 milliseconds = **0.04 seconds**. * **Calculation:** $1 / 0.04 = \mathbf{25\text{ Hz}}$. * At 25 Hz, the interval between stimuli is exactly equal to the twitch duration. Any frequency higher than this will result in **complete (fused) tetanus**. #### **2. Analysis of Incorrect Options** * **A (10 Hz) & B (20 Hz):** At these frequencies, the interval between stimuli (100 ms and 50 ms, respectively) is longer than the twitch duration (40 ms). This allows the muscle to partially relax, resulting in **incomplete (unfused) tetanus** or clonus. * **D (40 Hz):** While 40 Hz would certainly produce a fused contraction, it is not the **minimum** frequency required. The question specifically asks for the threshold frequency. #### **3. High-Yield Clinical Pearls (INI-CET/NEET-PG)** * **Treppe (Staircase Phenomenon):** When a muscle is stimulated repeatedly at low frequencies, the tension increases with each twitch due to a rise in cytosolic $Ca^{2+}$ and "warming up" of the muscle. * **Refractory Period:** Skeletal muscle has a very short refractory period (approx. 5 ms) compared to the twitch duration, which allows for **summation** and tetanus. * **Cardiac Muscle:** Cannot be tetanized because its refractory period is almost as long as its contraction period (approx. 200-300 ms), providing a vital protective mechanism against arrhythmias. * **Fast vs. Slow Twitch:** Fast-twitch fibers (Type II) have shorter twitch durations and thus require **higher** frequencies for tetanization compared to slow-twitch fibers (Type I).
Question 2: Which type of pulse is based on the Frank-Starling law?
- A. Pulsus parvus
- B. Pulsus alternans (Correct Answer)
- C. Pulsus bisferiens
- D. Pulsus paradoxus
Explanation: ### Explanation: Pulsus Alternans and the Frank-Starling Law **Pulsus alternans** is characterized by a regular rhythm but with alternating strong and weak beats. This phenomenon is a classic sign of **severe left ventricular systolic dysfunction** (Heart Failure). **The Mechanism (Frank-Starling Law):** The underlying physiology relies on the **Frank-Starling Law**, which states that the force of ventricular contraction is proportional to the initial length of the muscle fiber (End-Diastolic Volume). 1. **Weak Beat:** In a failing heart, a contraction may be weak, leading to incomplete emptying and a high **Residual Volume**. 2. **Strong Beat:** This high residual volume, combined with normal venous return, increases the **End-Diastolic Volume (Preload)** for the next beat. According to Frank-Starling, this increased stretch results in a more forceful contraction, creating a strong pulse. 3. The cycle then repeats, alternating between poor contractility and compensatory stretch. --- ### Analysis of Incorrect Options: * **A. Pulsus parvus:** Refers to a small-amplitude pulse. When combined with a delayed peak (*Pulsus parvus et tardus*), it is the hallmark of **Aortic Stenosis**. * **C. Pulsus bisferiens:** Characterized by two systolic peaks. It is typically seen in **Aortic Regurgitation** or **HOCM**. * **D. Pulsus paradoxus:** Defined as an exaggerated fall in systolic BP (>10 mmHg) during inspiration. It is a classic finding in **Cardiac Tamponade**. --- ### High-Yield Clinical Pearls for NEET-PG/INI-CET: * **Best site to palpate Pulsus alternans:** The **Radial artery** (it is easier to distinguish the alternating pressure changes in peripheral arteries). * **Electrical Alternans:** Do not confuse Pulsus alternans with Electrical alternans (alternating QRS amplitude on ECG), which is pathognomonic for **Pericardial Effusion/Tamponade**. * **Clinical Tip:** Pulsus alternans is often best elicited by applying light pressure with the sphygmomanometer cuff just below systolic pressure.
Question 3: Which of the following tissues will not be able to take up glucose in insulin resistance/insulin absence/diabetes mellitus?
- A. Kidney
- B. Skeletal muscle (Correct Answer)
- C. Brain
- D. Red blood cells
Explanation: ### **Explanation** The uptake of glucose into cells is mediated by **Glucose Transporters (GLUT)**. The fundamental concept here is the distinction between **Insulin-Dependent** and **Insulin-Independent** glucose uptake. **1. Why Skeletal Muscle is Correct:** * **Skeletal muscle** (and adipose tissue) primarily utilizes **GLUT-4**, which is the only **insulin-dependent** glucose transporter. * In the absence of insulin or in states of insulin resistance, GLUT-4 remains sequestered in intracellular vesicles and cannot translocate to the cell membrane. Consequently, these tissues cannot take up glucose effectively, leading to post-prandial hyperglycemia. **2. Why the Other Options are Incorrect:** * **Kidney (Option A):** Uses **GLUT-2** (basolateral membrane) and **SGLT-1/2** (apical membrane) for glucose reabsorption. These are insulin-independent. * **Brain (Option C):** Relies on **GLUT-1** and **GLUT-3**. The brain requires a continuous supply of glucose regardless of insulin levels to maintain metabolic function. * **Red Blood Cells (Option D):** Utilize **GLUT-1** for constitutive glucose uptake, which does not require insulin. --- ### **High-Yield Clinical Pearls for INI-CET** * **GLUT-4 Locations:** Remember the "Big Three": **Skeletal muscle**, **Cardiac muscle**, and **Adipose tissue**. * **Exercise Exception:** During exercise, skeletal muscle can take up glucose **independently of insulin** because muscle contraction triggers GLUT-4 translocation via the **AMPK pathway**. This is why exercise is a key management strategy for Type 2 Diabetes. * **GLUT-2:** Found in the **Liver, Pancreatic beta cells, and Small Intestine**. It acts as a "glucose sensor" due to its high $K_m$ (low affinity). * **SGLT:** These are **Secondary Active Transporters** (Sodium-Glucose Linked Transporters) found in the kidneys and small intestine, not to be confused with the facilitated diffusion of GLUTs.
Question 4: Which of the following is not a mechanism of action of ADH?
- A. Increases absorption of NaCl in thin ascending limb
- B. Increases water permeability in collecting ducts
- C. Increases absorption of urea in medullary collecting duct
- D. Increases absorption of urea in descending limb of loop of Henle (Correct Answer)
Explanation: ### **Explanation: Mechanism of Action of ADH (Vasopressin)** The correct answer is **D**, as ADH does **not** increase urea absorption in the descending limb of the loop of Henle. #### **1. Why Option D is Correct (The Concept)** Antidiuretic Hormone (ADH) acts primarily on the distal nephron to conserve water and maintain the medullary osmotic gradient. While ADH significantly increases **urea recycling**, it does so by increasing the expression of **UT-A1 and UT-A3 transporters** specifically in the **inner medullary collecting duct (IMCD)**. The **descending limb** of the loop of Henle is highly permeable to water but has low permeability to urea; ADH has no physiological effect on urea transport in this specific segment. #### **2. Analysis of Incorrect Options** * **Option A:** ADH stimulates the **NKCC2 transporter** in the **Thick Ascending Limb (TAL)**. This increases NaCl reabsorption, which strengthens the medullary osmotic gradient (countercurrent multiplication). * **Option B:** This is the classic action of ADH. It binds to **V2 receptors**, increasing cAMP, which leads to the insertion of **Aquaporin-2 (AQP2)** channels into the apical membrane of the **principal cells** in the collecting ducts. * **Option C:** As mentioned above, ADH increases urea reabsorption in the **medullary collecting duct**. This urea then enters the medullary interstitium, contributing nearly 50% of the hyperosmolarity required for concentrated urine. #### **3. High-Yield Clinical Pearls for INI-CET** * **V1 Receptors:** Located on vascular smooth muscle; cause **vasoconstriction** (Gq protein-coupled). * **V2 Receptors:** Located on renal tubular cells; cause **water reabsorption** (Gs protein-coupled). * **SIADH:** Characterized by excessive ADH, leading to **euvolemic hyponatremia** and highly concentrated urine. * **Diabetes Insipidus:** Central (lack of ADH) or Nephrogenic (resistance to ADH), resulting in massive polyuria and low urine osmolarity.
Question 5: Patient presents to OPD with fever. Which area is most likely involved?
- A. Periventricular hypothalamus
- B. Pre-optic nucleus (Correct Answer)
- C. Dorsomedial hypothalamus
- D. Insular cortex
Explanation: ### **Explanation: Thermoregulation and the Pre-optic Nucleus** The **Pre-optic Nucleus (PON)**, located in the anterior hypothalamus, is the body’s primary **thermostat**. It contains thermosensitive neurons that monitor blood temperature and receive input from peripheral receptors. #### **Why the Pre-optic Nucleus is Correct:** * **Central Integration:** The PON integrates thermal information to maintain the body's "set-point." * **Mechanism of Fever:** During a fever, exogenous pyrogens (like bacteria) trigger endogenous pyrogens (IL-1, IL-6, TNF). These induce **Prostaglandin E2 (PGE2)** synthesis in the hypothalamus. PGE2 acts directly on the **Pre-optic Area**, raising the hypothalamic set-point, leading to heat conservation and fever. #### **Analysis of Incorrect Options:** * **Periventricular Hypothalamus:** Primarily involved in the synthesis of releasing hormones (like TRH and CRH) and somatostatin; it does not play a direct role in temperature regulation. * **Dorsomedial Hypothalamus:** Involved in emotional behavior, blood pressure regulation, and gastrointestinal stimulation, but not the primary center for fever. * **Insular Cortex:** Involved in interoceptive awareness, gustation, and visceral sensations, but it is a cortical structure, not the primary hypothalamic regulator of temperature. --- ### **High-Yield Clinical Pearls for INI-CET:** * **Anterior Hypothalamus (Pre-optic):** Regulates response to **HEAT**. Lesion leads to **Hyperthermia**. (*Mnemonic: A/C = Anterior/Cooling*). * **Posterior Hypothalamus:** Regulates response to **COLD** (shivering). Lesion leads to **Poikilothermia** (inability to regulate temperature). * **Pyrogen Pathway:** The **OVLT** (Organum Vasculosum of the Lamina Terminalis) is the specific fenestrated capillary site where pyrogens enter the brain to trigger the PGE2 response. * **Antipyretics:** Drugs like Paracetamol and NSAIDs work by inhibiting **Cyclooxygenase (COX)**, thereby reducing PGE2 levels in the Pre-optic area.
Question 6: Which of the following is an example of feed forward mechanism?
- A. Temperature regulation (Correct Answer)
- B. Vasoconstriction in response to cooling
- C. Increase in cardiac output in response to anemia
- D. HR increases from supine to standing
Explanation: ### Explanation: Feed-Forward Mechanisms in Physiology **Feed-forward control** is a proactive regulatory mechanism where the body anticipates a change before it actually occurs in the internal environment. Unlike negative feedback, which reacts to a deviation from a set point, feed-forward mechanisms initiate a response to **prevent** a disturbance. #### Why "Temperature Regulation" is the Correct Answer: In the context of temperature regulation, **peripheral thermoreceptors** in the skin detect a change in environmental temperature (e.g., stepping into a cold room) and signal the hypothalamus **before** the core body temperature actually drops. This allows the body to initiate heat-saving measures (like shivering or non-shivering thermogenesis) in anticipation, maintaining a stable core temperature. #### Analysis of Incorrect Options: * **B. Vasoconstriction in response to cooling:** This is a **Negative Feedback** mechanism. The body has already detected a drop in temperature and is reacting to correct it. * **C. Increase in cardiac output in response to anemia:** This is a **Compensatory (Negative Feedback)** mechanism. The decrease in oxygen-carrying capacity triggers a reflex increase in heart rate and stroke volume to restore oxygen delivery to tissues. * **D. HR increases from supine to standing:** This is the **Baroreceptor Reflex**, a classic example of **Negative Feedback**. The drop in blood pressure upon standing (orthostasis) is sensed by baroreceptors, which then trigger an increase in heart rate to restore BP. --- ### High-Yield Clinical Pearls for INI-CET: * **Key Examples of Feed-Forward:** * **Cephalic phase of digestion:** Seeing or smelling food triggers insulin and gastric acid secretion before food enters the stomach. * **Exercise anticipation:** Increase in heart rate and ventilation *before* physical activity begins (mediated by the cerebral cortex). * **Distinction:** **Negative Feedback** is the most common homeostatic mechanism (e.g., hormonal axes, BP control). **Positive Feedback** is rare and often leads to an "explosive" event (e.g., LH surge, Oxytocin in labor, Blood clotting). * **Adaptive Control:** A complex form of feed-forward control occurs in the **Cerebellum** to coordinate rapid movements where sensory feedback is too slow.
Question 7: In a normal awake person at rest with eyes closed, EEG waves that are reduced on opening the eyes:
- A. Theta waves
- B. Beta waves
- C. Alpha waves (Correct Answer)
- D. Delta waves
Explanation: ### **Explanation** The correct answer is **Alpha waves**. #### **1. Why Alpha Waves are Correct** * **Alpha waves (8–13 Hz)** are the characteristic rhythm of a **relaxed, awake adult** with **eyes closed**. They are most prominent in the **parieto-occipital** regions. * The phenomenon described in the question is known as **Alpha Block** or **Desynchronization**. When the person opens their eyes or engages in focused mental activity (like solving a math problem), the synchronized alpha rhythm is replaced by fast, irregular, low-voltage activity. This happens because visual input "breaks" the synchronized firing of cortical neurons. #### **2. Why Other Options are Incorrect** * **Beta waves (13–30 Hz):** These are seen during **active thinking**, alertness, or tension. Opening the eyes usually *increases* beta activity rather than reducing it. * **Theta waves (4–7 Hz):** These are normal in **children** or during **Stage N1 sleep** in adults. In an awake adult, they may indicate emotional stress or certain brain disorders. * **Delta waves (<4 Hz):** These are the slowest waves with the highest amplitude. They are characteristic of **deep sleep (Stage N3)** or infancy. Their presence in an awake adult signifies serious organic brain disease. #### **3. High-Yield Clinical Pearls for INI-CET** * **Mnemonic for EEG Frequencies:** **D**on't **T**ouch **A**ll **B**uttons (**D**elta < **T**heta < **A**lpha < **B**eta). * **Alpha Block** is also called the **Berger Effect**, named after Hans Berger, the father of electroencephalography. * **Hyperventilation** is a common provocative test during EEG to bring out latent abnormalities (like 3 Hz spike-and-wave patterns in **Absence Seizures**). * **Brain Death:** Defined by a "flat" or **isoelectric EEG**, where no electrical activity >2 microvolts is recorded.
Question 8: Which of the following are features of Bezold Jarisch reflex? 1. Bradycardia 2. Hypertension 3. Coronary vasodilation 4. Tachycardia
- A. 1,2,3
- B. 1,3,4
- C. All of the above
- D. 1,3 (Correct Answer)
Explanation: ### **Explanation: Bezold-Jarisch Reflex (BJR)** The **Bezold-Jarisch Reflex** is a cardio-inhibitory reflex originating from sensory receptors (chemoreceptors and mechanoreceptors) located in the **ventricular walls**, particularly the inferoposterior wall of the left ventricle. #### **Mechanism & Correct Features** When these receptors are stimulated by chemical substances (e.g., alkaloids, serotonin, contrast media) or mechanical triggers (e.g., severe hypovolemia, myocardial ischemia), signals are sent via **unmyelinated C-fibers** (vagal afferents) to the medulla. This results in: 1. **Bradycardia (Feature 1):** Increased parasympathetic (vagal) tone slows the heart rate. 2. **Hypotension:** Widespread peripheral vasodilation occurs due to decreased sympathetic outflow. 3. **Coronary Vasodilation (Feature 3):** A protective mechanism to improve myocardial perfusion during stress. #### **Why Other Options are Incorrect** * **Hypertension (Feature 2):** Incorrect. The reflex causes a profound **drop in blood pressure** (hypotension), not an increase. * **Tachycardia (Feature 4):** Incorrect. The reflex is characterized by **bradycardia**. It is often considered a "paradoxical" reflex because, in states of low blood volume, the body usually responds with tachycardia (Baroreceptor reflex); however, the BJR overrides this, causing the heart to slow down. --- ### **High-Yield Clinical Pearls for INI-CET** * **The Triad:** The classic BJR triad is **Apnea, Bradycardia, and Hypotension**. * **Clinical Trigger:** It is frequently seen during **Inferior Wall Myocardial Infarction (IWMI)** because the receptors are concentrated in the inferior wall of the LV. * **Anesthesia Link:** It is a common cause of sudden bradycardia and hypotension during **Spinal Anesthesia**, especially if the patient is dehydrated. * **Chemical Stimulants:** Veratridine, nicotine, and capsaicin are known to trigger this reflex experimentally.
Question 9: Surface tension of the fluid lining the alveoli increases during:
- A. Inspiration
- B. Standing
- C. Expiration (Correct Answer)
- D. Supine
Explanation: ### **Explanation** The correct answer is **Expiration**. The surface tension of the fluid lining the alveoli is primarily regulated by **Surfactant** (a mixture of phospholipids, mainly Dipalmitoylphosphatidylcholine - DPPC). * **The Mechanism:** Surfactant molecules are interspersed between water molecules at the alveolar air-liquid interface. Their concentration per unit area determines the surface tension. * **During Expiration:** As the lungs deflate, the surface area of the alveoli **decreases**. This causes the surfactant molecules to become **more crowded** and tightly packed together. This increased density significantly lowers surface tension, preventing alveolar collapse (atelectasis) at low lung volumes. * **During Inspiration:** As the alveoli expand, the surfactant molecules spread further apart (density decreases). Consequently, the **surface tension increases** as the "diluted" surfactant is less effective at counteracting the cohesive forces of water. --- ### **Why Other Options are Incorrect** * **Inspiration:** As explained above, surface tension actually **increases** during inspiration because the surfactant molecules are spread over a larger surface area. * **Standing & Supine:** These represent **postural changes**. While posture affects regional ventilation-perfusion (V/Q) ratios and functional residual capacity (FRC) due to gravity, it does not directly alter the molecular density of surfactant or the intrinsic surface tension of the alveolar lining fluid. --- ### **High-Yield Clinical Pearls for INI-CET** * **Laplace’s Law:** $P = 2T / r$ (where $P$ is collapsing pressure, $T$ is surface tension, and $r$ is radius). Surfactant prevents small alveoli from collapsing into large ones by reducing $T$ as $r$ decreases. * **Source:** Surfactant is synthesized by **Type II Pneumocytes**. * **Composition:** The most important component is **DPPC (Lecithin)**. * **Clinical Correlation:** **Infant Respiratory Distress Syndrome (IRDS)** occurs in premature neonates due to surfactant deficiency, leading to high surface tension, stiff lungs, and widespread atelectasis. * **Hysteresis:** The difference between the inspiratory and expiratory compliance curves on a P-V loop is largely due to the time-dependent changes in surface tension.
Question 10: Which of the following is seen in high altitude?
- A. Respiratory alkalosis (Correct Answer)
- B. Respiratory acidosis
- C. Metabolic acidosis
- D. Metabolic alkalosis
Explanation: ### Explanation: Physiological Changes at High Altitude At high altitudes, the **barometric pressure decreases**, leading to a reduction in the **partial pressure of inspired oxygen ($PiO_2$)**. This creates a state of **hypobaric hypoxia**. #### Why Respiratory Alkalosis is Correct: 1. **Hypoxic Ventilatory Response:** The low $PaO_2$ is sensed by **peripheral chemoreceptors** (primarily the carotid bodies). 2. **Hyperventilation:** These receptors signal the brain to increase the rate and depth of breathing to improve oxygenation. 3. **CO2 Washout:** Excessive breathing causes the rapid elimination of carbon dioxide ($CO_2$). 4. **Alkalosis:** According to the Henderson-Hasselbalch equation, a decrease in $PaCO_2$ (hypocapnia) leads to an increase in blood pH, resulting in **Respiratory Alkalosis**. #### Why Other Options are Incorrect: * **Respiratory Acidosis:** This occurs during hypoventilation or CO2 retention (e.g., COPD), which is the opposite of the physiological response to altitude. * **Metabolic Acidosis:** While the kidneys eventually compensate for altitude by excreting bicarbonate ($HCO_3^-$), the primary and immediate change is respiratory. * **Metabolic Alkalosis:** This is typically seen in conditions like persistent vomiting or diuretic use, not as a primary response to hypoxia. --- ### High-Yield Facts for INI-CET: * **Renal Compensation:** After 24–48 hours at altitude, the kidneys compensate for respiratory alkalosis by **decreasing $HCO_3^-$ reabsorption** (causing a compensatory metabolic acidosis to normalize pH). * **Oxygen-Dissociation Curve:** Initially, alkalosis shifts the curve to the **left** (increasing $O_2$ affinity). Later, an increase in **2,3-BPG** shifts the curve back to the **right** to facilitate oxygen unloading at tissues. * **Acetazolamide:** This drug is used for **Acute Mountain Sickness (AMS)** because it inhibits carbonic anhydrase, forcing bicarbonate excretion and creating a mild metabolic acidosis that stimulates ventilation. * **Pulmonary Circulation:** Unlike systemic vessels, pulmonary vessels undergo **hypoxic pulmonary vasoconstriction**, which can lead to High-Altitude Pulmonary Edema (HAPE).