Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 171: Which of the following helps to create a high medullary osmotic gradient?

A. Thick ascending loop of Henle (Correct Answer)
B. Vasa recta
C. Counter-current exchanger system
D. All of the above

Explanation: The medullary osmotic gradient is essential for the kidney's ability to concentrate urine. Understanding the distinction between **creating** (establishing) and **maintaining** this gradient is high-yield for NEET-PG. ### 1. Why Option A is Correct The **Thick Ascending Limb (TAL)** of the Loop of Henle is the **"Single Effect"** or the "Engine" of the counter-current system. It actively transports Na⁺, K⁺, and 2Cl⁻ (via the NKCC2 transporter) from the tubular lumen into the medullary interstitium. Because the TAL is **impermeable to water**, it increases the osmolarity of the interstitium without water following the solutes. This active process is the primary step that **creates** the hypertonic medullary gradient. ### 2. Why Other Options are Incorrect * **Vasa Recta (Option B):** These vessels act as **Counter-current Exchangers**. They do not create the gradient; they **maintain** it by removing excess water and reabsorbing solutes in a way that prevents the "washout" of the medullary hypertonicity. * **Counter-current Exchanger System (Option C):** This refers specifically to the passive exchange mechanism (primarily in the vasa recta). While it is vital for preserving the gradient, it is a passive process and cannot establish the gradient from scratch. * **Option D:** Incorrect because only the active "multiplier" (TAL) creates the gradient. ### 3. High-Yield Clinical Pearls * **Counter-current Multiplier:** Refers to the Loop of Henle (creates the gradient). * **Counter-current Exchanger:** Refers to the Vasa Recta (maintains the gradient). * **Urea Recycling:** Contributes nearly 50% of the medullary osmotic gradient, particularly in the inner medulla. * **Loop Diuretics (e.g., Furosemide):** Act by inhibiting the NKCC2 transporter in the TAL, thereby "washing out" the medullary gradient and preventing urine concentration.

Question 172: All regulation of renal potassium excretion and total body potassium balance occurs in which part of the nephron?

A. Proximal convoluted tubule
B. Ascending loop of Henle
C. Descending loop of Henle
D. Collecting duct (Correct Answer)

Explanation: ### Explanation The regulation of potassium ($K^+$) balance is a critical function of the kidney. While the majority of filtered $K^+$ is reabsorbed in the early segments of the nephron, the **fine-tuning and ultimate regulation** of $K^+$ excretion occur in the **Late Distal Tubule and the Collecting Duct**. #### Why the Collecting Duct is Correct In the **Principal cells** of the collecting duct, $K^+$ is secreted into the tubular lumen. This process is highly regulated by **Aldosterone**, which increases the activity of Na+/K+ ATPase pumps and the number of apical $K^+$ channels (ROMK). When body $K^+$ levels are high, secretion increases; when levels are low, **Intercalated cells** can actively reabsorb $K^+$. This ability to switch between secretion and reabsorption makes the collecting duct the primary site for physiological regulation. #### Why Other Options are Incorrect * **Proximal Convoluted Tubule (PCT):** Approximately 65-70% of filtered $K^+$ is reabsorbed here, mainly via passive paracellular diffusion. This process is "obligatory" and does not change in response to the body's $K^+$ needs. * **Ascending Loop of Henle:** About 25% of $K^+$ is reabsorbed here via the **Na-K-2Cl symporter (NKCC2)**. Like the PCT, this is a fixed process and not the site of systemic regulation. * **Descending Loop of Henle:** This segment is primarily involved in water reabsorption and has minimal involvement in $K^+$ transport. #### High-Yield Clinical Pearls for NEET-PG * **Aldosterone Paradox:** Aldosterone acts on Principal cells to reabsorb $Na^+$ and secrete $K^+$. * **Diuretics:** Loop diuretics and Thiazides increase $K^+$ excretion (causing hypokalemia) by increasing flow rate and $Na^+$ delivery to the collecting duct. * **Hyperkalemia:** In renal failure, the inability of the collecting duct to secrete $K^+$ is a life-threatening complication. * **Intercalated Cells (Type A):** These are responsible for $K^+$ reabsorption during $K^+$ depletion via the H+/K+ ATPase pump.

Question 173: Which of the following is NOT a function of angiotensin?

A. Stimulates aldosterone secretion from the adrenal cortex
B. Increases renal blood flow (RBF)
C. Directly causes vasoconstriction
D. Acts on the adrenal medulla and stimulates steroidogenesis (Correct Answer)

Explanation: ### Explanation The correct answer is **D: Acts on the adrenal medulla and stimulates steroidogenesis.** #### Why Option D is Correct Angiotensin II (AT-II) acts on the **adrenal cortex**, specifically the **zona glomerulosa**, to stimulate the synthesis and secretion of **aldosterone** (a mineralocorticoid). It does **not** act on the adrenal medulla, which is responsible for secreting catecholamines (epinephrine and norepinephrine). Furthermore, steroidogenesis occurs in the cortex, not the medulla. #### Analysis of Other Options * **Option A:** AT-II is the primary stimulus for **aldosterone secretion**. Aldosterone then acts on the principal cells of the late distal tubule and collecting duct to increase sodium reabsorption and potassium secretion. * **Option B:** AT-II is a potent **vasoconstrictor**. In the kidneys, it preferentially constricts the **efferent arteriole** more than the afferent arteriole. While this helps maintain the Glomerular Filtration Rate (GFR) during low-pressure states, the overall effect of systemic vasoconstriction and renal arteriolar constriction is a **decrease in Renal Blood Flow (RBF)**. Therefore, "increasing RBF" is generally incorrect, but in the context of this question, Option D is a more blatant physiological error. * **Option C:** AT-II is one of the most powerful direct **vasoconstrictors** in the body. It acts via AT1 receptors on vascular smooth muscle to increase systemic vascular resistance and blood pressure. #### NEET-PG High-Yield Pearls * **Site of Action:** AT-II acts on **AT1 receptors** (Gq protein-coupled) to mediate most of its known effects (vasoconstriction, thirst, aldosterone release). * **Renal Autoregulation:** At low concentrations, AT-II maintains GFR by constricting the efferent arteriole (increasing filtration fraction). * **Brain Effect:** It acts on the **subfornical organ** to stimulate the thirst center and increases ADH secretion from the posterior pituitary. * **ACE Inhibitors:** These drugs block the conversion of AT-I to AT-II, leading to vasodilation and decreased aldosterone, making them first-line for hypertension and heart failure.

Question 174: Which of the following hormones is not secreted by the kidney?

A. 1,25-dihydroxycholecalciferol
B. Angiotensin I (Correct Answer)
C. Renin
D. Erythropoietin

Explanation: ### Explanation The kidney is a vital endocrine organ, but it does not directly secrete **Angiotensin I**. **Why Angiotensin I is the correct answer:** Angiotensin I is not secreted by any gland; rather, it is **formed in the plasma**. The process begins when the kidneys secrete the enzyme **Renin** into the bloodstream. Renin then acts on **Angiotensinogen** (a plasma protein synthesized by the **liver**) to cleave it into Angiotensin I. Subsequently, Angiotensin I is converted to Angiotensin II by the Angiotensin-Converting Enzyme (ACE), primarily in the lungs. **Why the other options are incorrect:** * **1,25-dihydroxycholecalciferol (Calcitriol):** This is the active form of Vitamin D. The final step of its activation (hydroxylation of 25-hydroxyvitamin D) occurs in the **proximal convoluted tubules** of the kidney via the enzyme 1-alpha-hydroxylase. * **Renin:** Produced and secreted by the **Juxtaglomerular (JG) cells** of the afferent arteriole in response to low blood pressure or low sodium delivery. * **Erythropoietin (EPO):** Approximately 85-90% of EPO is secreted by the **interstitial cells of the peritubular capillary bed** in the renal cortex in response to hypoxia. **High-Yield Clinical Pearls for NEET-PG:** * **Site of EPO production:** In adults, it is the kidney; in the fetus, it is primarily the **liver**. * **Thrombopoietin:** While mainly produced in the liver, the kidney also produces a small amount. * **Prostaglandins:** The kidney also secretes PGE2 and PGI2, which act as local vasodilators to maintain renal blood flow.

Question 175: Maximum amount of glucose absorption occurs at which part of the nephron?

A. Proximal Convoluted Tubule (PCT) (Correct Answer)
B. Distal Convoluted Tubule (DCT)
C. Loop of Henle
D. None of the above

Explanation: **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for the reabsorption of essential solutes. Under normal physiological conditions, **100% of filtered glucose** is reabsorbed in the PCT, ensuring that no glucose appears in the urine. **Why PCT is the correct answer:** Glucose reabsorption in the PCT occurs via **secondary active transport**. * **SGLT-2 (Sodium-Glucose Co-transporter 2):** Located in the early segment (S1) of the PCT, it reabsorbs about 90% of the glucose. * **SGLT-1:** Located in the later segment (S3) of the PCT, it reabsorbs the remaining 10%. * Once inside the tubular cell, glucose exits into the blood via facilitated diffusion through **GLUT-2** (early PCT) and **GLUT-1** (late PCT) transporters. **Why other options are incorrect:** * **Loop of Henle:** This segment is primarily involved in the concentration of urine and the reabsorption of water (descending limb) and electrolytes like Na⁺, K⁺, and Cl⁻ (thick ascending limb). It does not possess glucose transporters. * **Distal Convoluted Tubule (DCT):** The DCT is involved in the fine-tuning of electrolytes (Na⁺, Ca²⁺) and acid-base balance. By the time tubular fluid reaches the DCT, all glucose should have already been reabsorbed. **High-Yield NEET-PG Pearls:** 1. **Renal Threshold for Glucose:** Glucose starts appearing in the urine (glycosuria) when blood glucose levels exceed **180 mg/dL**. 2. **Transport Maximum (TmG):** The point at which all glucose transporters are saturated. In men, it is approximately **375 mg/min**; in women, **300 mg/min**. 3. **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A modern class of anti-diabetic drugs that work by inhibiting glucose reabsorption in the PCT, promoting its excretion in urine.

Question 176: Antidiuretic hormone (ADH) primarily acts on which part of the nephron?

A. Proximal convoluted tubule
B. Loop of Henle
C. Collecting duct (Correct Answer)
D. All parts of the nephron

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Antidiuretic Hormone (ADH), also known as Vasopressin, is the primary regulator of water balance in the body. Its main site of action is the **Collecting Duct** (specifically the principal cells). ADH binds to **V2 receptors** on the basolateral membrane, triggering a cAMP-mediated signaling pathway. This leads to the insertion of **Aquaporin-2 (AQP2)** water channels into the apical membrane. This process increases the water permeability of the collecting duct, allowing water to be reabsorbed down the osmotic gradient into the hypertonic medullary interstitium, resulting in concentrated urine. **2. Why the Other Options are Wrong:** * **Proximal Convoluted Tubule (PCT):** Reabsorption here is "obligatory" and occurs regardless of ADH levels. About 65% of water is reabsorbed here via Aquaporin-1. * **Loop of Henle:** The descending limb is permeable to water, and the ascending limb is impermeable. While ADH can increase NaCl reabsorption in the Thick Ascending Limb (TAL) to enhance medullary hypertonicity, this is not its *primary* site for water regulation. * **All parts of the nephron:** ADH action is highly specific to the distal segments (late distal tubule and collecting ducts). Most of the nephron is either constitutively permeable or entirely impermeable to water independent of ADH. **3. High-Yield Clinical Pearls for NEET-PG:** * **V1 Receptors:** Located on vascular smooth muscle; cause vasoconstriction (hence the name Vasopressin). * **Diabetes Insipidus (DI):** Deficiency of ADH (Central DI) or resistance to its action (Nephrogenic DI) leads to polyuria and dilute urine. * **SIADH:** Excessive ADH secretion leads to water retention and dilutional hyponatremia. * **Urea Recycling:** ADH also increases the permeability of the **medullary** collecting duct to urea (via UT-A1 transporters), which is crucial for maintaining the corticomedullary osmotic gradient.

Question 177: In a person on a very low potassium diet, which part of the nephron would be expected to reabsorb the most potassium?

A. Proximal convoluted tubule (Correct Answer)
B. Loop of Henle
C. Distal convoluted tubule
D. Collecting duct

Explanation: ### Explanation The correct answer is **A. Proximal convoluted tubule (PCT)**. **Why the Proximal Convoluted Tubule is Correct:** Regardless of dietary potassium intake (whether high, normal, or very low), the **Proximal Convoluted Tubule** is always the primary site of potassium reabsorption. Approximately **65-70%** of filtered potassium is reabsorbed here, primarily via passive paracellular mechanisms (solvent drag and diffusion). This process is "obligatory," meaning it occurs independently of the body's potassium balance or hormonal influence. **Why the Other Options are Incorrect:** * **B. Loop of Henle:** This site reabsorbs about **25-30%** of filtered potassium, mainly via the **NKCC2 cotransporter** in the Thick Ascending Limb (TAL). While significant, it is quantitatively less than the PCT. * **C. Distal Convoluted Tubule (DCT):** The DCT plays a minor role in potassium handling compared to the proximal segments. * **D. Collecting Duct:** This is the site of **facultative** (regulated) potassium handling. In a low-potassium diet, the Alpha-Intercalated cells reabsorb potassium via H⁺-K⁺ ATPase. However, even under maximal stimulation, the total amount reabsorbed here is much smaller than the bulk reabsorption that occurs in the PCT. **NEET-PG High-Yield Pearls:** 1. **Fixed vs. Variable:** The PCT and Loop of Henle reabsorb a fixed percentage (~90-95% total) of filtered K⁺. The **Late Distal Tubule and Collecting Duct** are the sites that determine the final urinary excretion based on dietary needs. 2. **Secretory Site:** In a **high-potassium diet**, the Principal cells of the collecting duct secrete K⁺ into the lumen under the influence of **Aldosterone**. 3. **Low K⁺ Diet:** In states of depletion, secretion by Principal cells is inhibited, and reabsorption by **Alpha-Intercalated cells** is activated. 4. **Summary Rule:** For almost all electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻) and water, the **PCT** is the site of maximum reabsorption.

Question 178: What is true about the autoregulation of renal blood flow?

A. Changes are proportional to MAP
B. Changes are inversely proportional to changes in MAP
C. Remains relatively constant over changes in MAP (Correct Answer)
D. None of the above

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Autoregulation is the intrinsic ability of the kidney to maintain a **relatively constant Renal Blood Flow (RBF) and Glomerular Filtration Rate (GFR)** despite fluctuations in Mean Arterial Pressure (MAP). This occurs between a physiological range of approximately **80 to 180 mmHg**. The underlying mechanism involves two primary processes: * **Myogenic Mechanism:** The afferent arteriole contracts in response to stretch (increased pressure) and dilates when pressure falls, maintaining constant flow. * **Tubuloglomerular Feedback (TGF):** The Macula Densa senses changes in NaCl delivery; increased flow leads to the release of adenosine, causing afferent arteriolar vasoconstriction to normalize GFR. **2. Why the Incorrect Options are Wrong:** * **Option A & B:** If RBF were directly or inversely proportional to MAP, the kidneys would be vulnerable to massive fluctuations in filtration and potential damage during hypertension, or acute renal failure during mild hypotension. Autoregulation ensures that the kidney is "uncoupled" from systemic blood pressure variations within the 80–180 mmHg window. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Range:** Autoregulation is most efficient between **80–180 mmHg**. Below 80 mmHg, RBF drops linearly with pressure. * **Independence:** Autoregulation persists even in denervated or isolated kidneys, proving it is an **intrinsic** intrarenal mechanism, not dependent on external nerves or hormones. * **Afferent vs. Efferent:** The **Afferent arteriole** is the primary site of resistance changes for autoregulation. * **Exception:** While RBF and GFR are autoregulated, **urine output is NOT**. It increases with MAP (Pressure Diuresis).

Question 179: All of the following electrolytes are absorbed in the distal convoluted tubule (DCT), EXCEPT:

A. Na+
B. K+ (Correct Answer)
C. Ca2+
D. Cl-

Explanation: ### Explanation The **Distal Convoluted Tubule (DCT)** is a crucial segment for the fine-tuning of electrolytes, accounting for about 5–10% of sodium and water reabsorption. **Why K+ is the correct answer:** In the DCT (specifically the late DCT) and the collecting duct, **Potassium (K+) is primarily secreted**, not absorbed. Under the influence of **Aldosterone**, Principal cells reabsorb Na+ and water while simultaneously secreting K+ into the tubular lumen to maintain electrical neutrality and potassium homeostasis. While some K+ reabsorption can occur in the intercalated cells during states of severe hypokalemia, the physiological hallmark of this segment is K+ secretion. **Analysis of Incorrect Options:** * **Na+ and Cl-:** These are absorbed together in the early DCT via the **NCC (Na+-Cl- cotransporter)**. This transporter is the specific target of **Thiazide diuretics**. * **Ca2+:** The DCT is the major site for the active, regulated reabsorption of Calcium. This process is stimulated by **Parathyroid Hormone (PTH)**, which increases the expression of apical calcium channels (TRPV5). **High-Yield Clinical Pearls for NEET-PG:** * **Thiazide Diuretics:** Inhibit the Na+-Cl- cotransporter in the DCT. A unique side effect is **hypercalcemia**, as thiazides enhance Ca2+ reabsorption in this segment. * **Gitelman Syndrome:** A genetic defect in the NCC transporter in the DCT, presenting with symptoms similar to chronic thiazide use (hypokalemia, metabolic alkalosis, and hypocalciuria). * **Macula Densa:** Located at the junction of the thick ascending limb and the DCT; it senses NaCl delivery to regulate the Glomerular Filtration Rate (GFR) via tubuloglomerular feedback.

Question 180: What is the action of renin?

A. Erythropoiesis
B. Controls secretion of melatonin
C. Converts angiotensinogen to angiotensin I (Correct Answer)
D. Converts angiotensinogen I to angiotensinogen II

Explanation: **Explanation:** The correct answer is **C. Converts angiotensinogen to angiotensin I.** Renin is a proteolytic enzyme (aspartyl protease) secreted by the **Juxtaglomerular (JG) cells** of the afferent arteriole in the kidney. It is the rate-limiting step of the **Renin-Angiotensin-Aldosterone System (RAAS)**. When blood pressure or sodium levels drop, renin is released into the bloodstream where it acts upon **Angiotensinogen** (a plasma protein synthesized by the liver) to cleave it into the decapeptide **Angiotensin I**. **Analysis of Incorrect Options:** * **Option A:** Erythropoiesis is stimulated by **Erythropoietin (EPO)**, which is produced by the interstitial cells of the peritubular capillary bed in the renal cortex, not by renin. * **Option B:** Melatonin secretion is controlled by the **Pineal gland** in response to light-dark cycles sensed by the suprachiasmatic nucleus (SCN) of the hypothalamus. * **Option D:** The conversion of Angiotensin I to Angiotensin II is catalyzed by **Angiotensin-Converting Enzyme (ACE)**, primarily located in the vascular endothelium of the lungs. **High-Yield Clinical Pearls for NEET-PG:** * **Stimuli for Renin Release:** 1. Decreased renal perfusion pressure (baroreceptors in afferent arteriole), 2. Decreased NaCl delivery to **Macula Densa**, 3. Sympathetic stimulation (via $\beta_1$ receptors). * **Inhibitor:** Renin release is inhibited by Atrial Natriuretic Peptide (ANP) and Angiotensin II (negative feedback). * **Drug Link:** **Aliskiren** is a direct renin inhibitor used in hypertension that prevents the conversion of angiotensinogen to angiotensin I.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

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Tubular Reabsorption and Secretion

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Concentration and Dilution of Urine

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Acid-Base Regulation by the Kidneys

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Sodium and Water Balance

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Potassium Regulation

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Calcium and Phosphate Handling

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Micturition Physiology

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Renal Function Tests

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Integrative Responses to Fluid Challenges

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