Renal Physiology Practice Questions

Q: Maximum potassium reabsorption occurs in which part of the nephron?

Proximal convoluted tubule. **Explanation:** The handling of potassium ($K^+$) by the kidney is unique because it involves both reabsorption and secretion. However, the vast majority of filtered potassium is reabsorbed in the early segments of the nephron, regardless of the body's potassium status. **1. Why Proximal Convoluted Tubule (PCT) is correct:** The **PCT is the primary site of potassium reabsorption**, accounting for approximately **65-70%** of the filtered load. This process is primarily passive and occurs via the **paracellular route**. It is driven by solvent drag (as water is reabsorbed) and the positive transtubular potential that develops in the late PCT. An additional 25-30% is reabsorbed in the Thick Ascending Limb (TAL) via the Na-K-2Cl cotransporter. **2. Why the other options are incorrect:** * **Distal Convoluted Tubule (DCT) & Collecting Ducts:** These segments are responsible for the **fine-tuning** of potassium balance. While some reabsorption can occur here via α-intercalated cells (using H-K ATPase) during potassium depletion, these segments are more clinically significant for **potassium secretion**. * **Cortical Collecting Duct:** This is the major site of **potassium secretion** regulated by Aldosterone. Principal cells secrete $K^+$ into the tubular lumen to maintain systemic balance. **3. High-Yield Clinical Pearls for NEET-PG:** * **Fixed vs. Variable:** $K^+$ reabsorption in the PCT and Loop of Henle is "fixed" (constant), whereas secretion in the late distal tubule/collecting duct is "variable" and regulated by dietary intake and hormones. * **Aldosterone Effect:** Aldosterone acts on the Principal cells of the collecting duct to increase $Na^+$ reabsorption and $K^+$ secretion. * **Diuretics:** Loop diuretics and Thiazides increase $K^+$ delivery to the distal nephron, leading to increased $K^+$ secretion and subsequent hypokalemia.

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

Thick ascending loop of Henle. 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.

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

Angiotensin I. ### 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.

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

Proximal Convoluted Tubule (PCT). **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.

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

Collecting duct. **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.

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

Remains relatively constant over changes in MAP. ### 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).

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

K+. ### 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 1

Maximum potassium reabsorption occurs in which part of the nephron?

Accepted Answer

Proximal convoluted tubule

Answer

Distal convoluted tubule

Answer

Cortical collecting duct

Answer

Medullary collecting duct

Question 2

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

Accepted Answer

Thick ascending loop of Henle

Answer

Vasa recta

Answer

Counter-current exchanger system

Answer

All of the above

Question 3

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

Accepted Answer

Collecting duct

Answer

Proximal convoluted tubule

Answer

Ascending loop of Henle

Answer

Descending loop of Henle

Question 4

Which of the following is NOT a function of angiotensin?

Accepted Answer

Acts on the adrenal medulla and stimulates steroidogenesis

Answer

Stimulates aldosterone secretion from the adrenal cortex

Answer

Increases renal blood flow (RBF)

Answer

Directly causes vasoconstriction

Question 5

Which of the following hormones is not secreted by the kidney?

Accepted Answer

Angiotensin I

Answer

1,25-dihydroxycholecalciferol

Answer

Renin

Answer

Erythropoietin

Question 6

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

Accepted Answer

Proximal Convoluted Tubule (PCT)

Answer

Distal Convoluted Tubule (DCT)

Answer

Loop of Henle

Answer

None of the above

Question 7

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

Accepted Answer

Collecting duct

Answer

Proximal convoluted tubule

Answer

Loop of Henle

Answer

All parts of the nephron

Question 8

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

Accepted Answer

Proximal convoluted tubule

Answer

Loop of Henle

Answer

Distal convoluted tubule

Answer

Collecting duct

Question 9

What is true about the autoregulation of renal blood flow?

Accepted Answer

Remains relatively constant over changes in MAP

Answer

Changes are proportional to MAP

Answer

Changes are inversely proportional to changes in MAP

Answer

None of the above

Question 10

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

Accepted Answer

K+

Answer

Na+

Answer

Ca2+

Answer

Cl-

Renal Physiology — MCQs

Renal Physiology — MCQs

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