Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 131: Erythropoietin is produced by which of the following?

A. Juxtaglomerular cells
B. Interstitial cells of the peritubular capillary bed of the kidneys (Correct Answer)
C. Pars recta of PCT
D. Macula densa

Explanation: ### Explanation **Correct Answer: B. Interstitial cells of the peritubular capillary bed of the kidneys** Erythropoietin (EPO) is a glycoprotein hormone essential for erythropoiesis. In adults, approximately **90% of EPO** is produced by the **interstitial cells (fibroblast-like cells)** located in the peritubular capillary bed of the renal cortex and outer medulla. These cells act as oxygen sensors; when they detect hypoxia (via Hypoxia-Inducible Factor - HIF-1α), they increase the transcription of the EPO gene. The remaining 10% is produced by the liver (perisinusoidal hepatocytes). **Analysis of Incorrect Options:** * **A. Juxtaglomerular (JG) cells:** These are modified smooth muscle cells located in the afferent arteriole. Their primary function is the secretion of **Renin** in response to low blood pressure or sympathetic stimulation, not EPO. * **C. Pars recta of PCT:** This is the straight portion of the proximal convoluted tubule. While the PCT is metabolically active and susceptible to ischemic injury, it is not the site of EPO synthesis. * **D. Macula densa:** These are specialized cells in the distal convoluted tubule that sense **sodium chloride (NaCl) concentration**. They provide feedback to the JG cells to regulate GFR and renin release (tubuloglomerular feedback). **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Kidney Disease (CKD):** Destruction of the renal interstitium leads to EPO deficiency, resulting in **normocytic normochromic anemia**. This is treated with recombinant human erythropoietin (Epoetin alfa). * **Fetal Life:** In the fetus, the **liver** is the primary site of EPO production; the switch to the kidneys occurs near birth. * **Polycythemia:** Ectopic EPO production can occur in certain tumors, most notably **Renal Cell Carcinoma (RCC)** and **Hepatocellular Carcinoma (HCC)**.

Question 132: What is true regarding neutral substances with a size of 6 nm?

A. Freely filtered
B. Permeability is inversely proportional to diameter (Correct Answer)
C. Not filtered
D. None of the above

Explanation: The glomerular filtration barrier acts as both a **size-selective** and **charge-selective** sieve. Understanding the dynamics of solute filtration is a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer** **Option B** is correct because the permeability of the glomerular capillary wall to neutral substances is determined primarily by molecular size (effective radius). For neutral solutes, as the molecular diameter increases, the filterability (clearance ratio) decreases. * Substances with a diameter **< 4 nm** are generally **freely filtered** (ratio = 1.0). * Substances with a diameter **> 8 nm** are **not filtered** (ratio = 0). * For substances in the intermediate range (**4 nm to 8 nm**), such as the 6 nm substance mentioned, permeability is **inversely proportional to the diameter**. As the size approaches the pore limit, the restriction increases. ### **Why Other Options are Incorrect** * **Option A (Freely filtered):** Only substances with a diameter less than 4 nm (e.g., water, glucose, urea, inulin) are freely filtered. A 6 nm substance is large enough to face significant hindrance. * **Option C (Not filtered):** Total restriction typically occurs at diameters exceeding 8 nm. A 6 nm neutral substance will still be filtered, albeit at a reduced rate compared to smaller molecules. ### **High-Yield Clinical Pearls for NEET-PG** 1. **Charge Selectivity:** The glomerular basement membrane and podocytes are coated with **heparan sulfate (polyanionic)**. Therefore, for any given size, **cations (+)** are filtered most easily, followed by **neutral** molecules, while **anions (-)** (like albumin) are repelled. 2. **Dextran Studies:** Researchers use dextrans of varying sizes and charges to map the permeability curve of the kidney. 3. **Minimal Change Disease (MCD):** In MCD, there is a loss of the negative charge (polyanionic barrier) on the glomerular wall. This leads to proteinuria (specifically albuminuria) even though the "pore size" remains unchanged.

Question 133: Isoosmolar urine is seen in which of the following conditions?

A. Acute tubular necrosis (ATN) (Correct Answer)
B. Severe dehydration
C. Diabetes insipidus
D. Polycystic kidney disease (PCKD)

Explanation: **Explanation:** The correct answer is **Acute Tubular Necrosis (ATN)**. **1. Why ATN is correct:** In ATN, the tubular epithelial cells are damaged, leading to a loss of the kidney's ability to concentrate or dilute urine. The kidneys cannot maintain the medullary osmotic gradient or respond to ADH. Consequently, the urine osmolality becomes fixed at the same osmolality as plasma (approximately **290–300 mOsm/kg**), a condition known as **Isosthenuria**. The Urine Specific Gravity typically remains fixed at **1.010**. **2. Why other options are incorrect:** * **Severe Dehydration:** In a healthy physiological response to dehydration, ADH levels rise, causing the kidneys to reabsorb water. This results in **hyperosmolar (concentrated) urine** (typically >500 mOsm/kg). * **Diabetes Insipidus (DI):** Whether central or nephrogenic, DI is characterized by an inability to reabsorb water in the collecting ducts. This leads to the excretion of large volumes of **hypoosmolar (dilute) urine** (typically <200 mOsm/kg). * **Polycystic Kidney Disease (PCKD):** While chronic kidney diseases eventually lead to a loss of concentrating ability, in the early to mid-stages, the urine is not characteristically isoosmolar in the same acute, diagnostic fashion as seen in ATN. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isosthenuria:** Urine Specific Gravity of **1.010** is the hallmark of ATN. * **Fractional Excretion of Sodium (FeNa):** In ATN, FeNa is typically **>2%** (due to tubular damage), whereas in Pre-renal Azotemia, it is **<1%** (tubules are intact and conserving sodium). * **Urinary Casts:** Look for **"Muddy brown granular casts"** in the urine sediment, which are pathognomonic for ATN.

Question 134: Aldosterone is known to cause sodium retention. Its sodium-retaining action is exerted on which part of the nephron?

A. Proximal convoluted tubule
B. Ascending limb of loop of Henle
C. Collecting ducts (Correct Answer)
D. Early distal convoluted tubule

Explanation: ### Explanation **Correct Answer: C. Collecting ducts** **Mechanism of Action:** Aldosterone is a mineralocorticoid synthesized in the *zona glomerulosa* of the adrenal cortex. Its primary site of action is the **Principal cells (P cells)** of the **late distal tubule** and, most importantly, the **cortical collecting ducts**. Aldosterone binds to intracellular mineralocorticoid receptors, leading to the up-regulation and de novo synthesis of: 1. **ENaC (Epithelial Sodium Channels)** on the apical membrane. 2. **Na+/K+ ATPase pumps** on the basolateral membrane. This results in increased sodium reabsorption into the blood and increased potassium secretion into the tubular lumen. **Analysis of Incorrect Options:** * **A. Proximal Convoluted Tubule (PCT):** This is the site of bulk reabsorption (65% of Na+). Sodium reabsorption here is primarily driven by the Na+/H+ exchanger (NHE3) and is influenced by Angiotensin II, not Aldosterone. * **B. Ascending limb of Loop of Henle:** The thick ascending limb (TAL) reabsorbs ~25% of Na+ via the **Na+-K+-2Cl- cotransporter (NKCC2)**. This area is the target for loop diuretics (e.g., Furosemide), not Aldosterone. * **D. Early Distal Convoluted Tubule:** This segment reabsorbs Na+ via the **Na+-Cl- cotransporter (NCC)**, which is the target for Thiazide diuretics. Aldosterone acts specifically on the *late* portion of the DT and the collecting ducts. **NEET-PG High-Yield Pearls:** * **Conn’s Syndrome:** Primary hyperaldosteronism characterized by HTN, hypokalemia, and metabolic alkalosis. * **Spironolactone/Eplerenone:** These are aldosterone antagonists (K+-sparing diuretics) that act on the collecting ducts. * **Liddle’s Syndrome:** A genetic mutation causing overactivity of ENaC channels in the collecting ducts, mimicking hyperaldosteronism but with low aldosterone levels. * **Escape Phenomenon:** In primary hyperaldosteronism, the body eventually limits sodium retention (preventing edema) due to ANP release, though potassium wasting continues.

Question 135: All of the following are true about the macula densa except:

A. It is a sensor for glomerulotubular balance. (Correct Answer)
B. Na+ and Cl- enter the macula densa cells via the Na-K-2Cl cotransporter in their apical membranes.
C. Increased formation and release of adenosine occurs in macula densa cells.
D. It causes vasoconstriction of the afferent arteriole.

Explanation: ### Explanation The **Macula Densa** is a specialized cluster of epithelial cells in the distal convoluted tubule that functions as the sensory component of **Tubuloglomerular Feedback (TGF)**, not Glomerulotubular Balance (GTB). **1. Why Option A is the Correct Answer (The False Statement):** * **Glomerulotubular Balance (GTB)** is an intrinsic property of the **proximal tubule** where a constant fraction of the filtered load is reabsorbed, regardless of GFR changes. It does not involve the macula densa. * The macula densa is the sensor for **Tubuloglomerular Feedback (TGF)**, a mechanism that regulates GFR by sensing NaCl concentration at the distal tubule and adjusting afferent arteriolar resistance. **2. Analysis of Other Options:** * **Option B:** Macula densa cells sense NaCl levels via the **NKCC2 (Na-K-2Cl) transporter** on their apical membranes. This is the rate-limiting step for the TGF signal. * **Option C:** When NaCl levels are high, increased transport leads to ATP consumption and the subsequent generation of **Adenosine**. Adenosine acts as the primary signaling molecule in TGF. * **Option D:** Adenosine binds to **A1 receptors** on the smooth muscle of the **afferent arteriole**, causing vasoconstriction. This reduces hydrostatic pressure and GFR to prevent excessive salt loss. **3. High-Yield Clinical Pearls for NEET-PG:** * **Location:** Macula densa is located at the transition between the Thick Ascending Limb (TAL) and the Distal Convoluted Tubule (DCT). * **Juxtaglomerular Apparatus (JGA):** Comprises the Macula densa, Juxtaglomerular (JG) cells (modified smooth muscle of afferent arteriole), and Lacis cells (extraglomerular mesangial cells). * **Inverse Relationship:** High NaCl at macula densa → **Decreased Renin** and **Afferent Vasoconstriction**. * **Drug Interaction:** Loop diuretics (Furosemide) inhibit NKCC2 in the macula densa, effectively "blunting" the TGF response.

Question 136: Which of the following conditions leads to a decrease in Glomerular Filtration Rate (GFR)?

A. Hypotension (Correct Answer)
B. Hypoproteinemia
C. Diuretics
D. All of the above

Explanation: **Explanation:** The Glomerular Filtration Rate (GFR) is determined by the balance of Starling forces across the glomerular capillaries. The formula for GFR is: **GFR = Kf × [(Pgc – Pbs) – (πgc – πbs)]**. **1. Why Hypotension is Correct:** Hypotension leads to a decrease in **Glomerular Capillary Hydrostatic Pressure (Pgc)**. Since Pgc is the primary driving force for filtration, a significant drop in systemic blood pressure (below the autoregulatory range of 80–180 mmHg) reduces the pressure gradient available to push fluid into Bowman’s space, thereby decreasing GFR. **2. Why the other options are incorrect:** * **Hypoproteinemia:** This condition involves a decrease in plasma proteins (e.g., albumin), which lowers the **Plasma Colloid Osmotic Pressure (πgc)**. Since πgc is a force that opposes filtration, a decrease in this pressure actually **increases GFR**. * **Diuretics:** Most diuretics act by inhibiting ion reabsorption in the tubules (e.g., Loop diuretics in the Thick Ascending Limb). While they increase urine output, they do not acutely decrease GFR as a primary mechanism; in fact, some (like Mannitol) may transiently increase GFR by expanding extracellular volume. **Clinical Pearls for NEET-PG:** * **Autoregulation:** GFR remains constant between mean arterial pressures of **80–180 mmHg** due to the Myogenic mechanism and Tubuloglomerular Feedback (TGF). * **Afferent vs. Efferent:** Constriction of the afferent arteriole decreases GFR, while moderate constriction of the efferent arteriole increases GFR. * **Gold Standard Marker:** Inulin clearance is the gold standard for measuring GFR because it is freely filtered but neither reabsorbed nor secreted.

Question 137: Liddle syndrome is caused by a genetic defect in which of the following?

A. Na-K-2Cl co-transporter in the thick ascending limb of the loop of Henle
B. Na-Cl cotransporter in the distal tubule
C. ENaC (epithelial sodium channel) in the collecting duct/tubule (Correct Answer)
D. Aquaporin 2 (AQP2) channel in the collecting duct

Explanation: **Explanation:** **Liddle Syndrome** is an autosomal dominant genetic disorder characterized by a "gain-of-function" mutation in the genes encoding the **ENaC (Epithelial Sodium Channel)** subunits. This mutation prevents the normal degradation of ENaC channels, leading to an increased number of these channels remaining active on the apical membrane of the principal cells in the **collecting duct**. 1. **Why Option C is Correct:** The persistent activation of ENaC leads to excessive sodium reabsorption and subsequent water retention. This results in hypertension, hypokalemia, and metabolic alkalosis. Crucially, because the body senses high volume, renin and aldosterone levels are suppressed (**Pseudohyperaldosteronism**). 2. **Why Other Options are Incorrect:** * **Option A:** Mutations in the Na-K-2Cl cotransporter (NKCC2) cause **Bartter Syndrome**, which mimics loop diuretic use (hypotension, hypokalemia). * **Option B:** Mutations in the Na-Cl cotransporter (NCC) cause **Gitelman Syndrome**, which mimics thiazide diuretic use. * **Option D:** Defects in Aquaporin 2 (AQP2) are associated with **Nephrogenic Diabetes Insipidus**, leading to polyuria and inability to concentrate urine. **NEET-PG High-Yield Pearls:** * **Clinical Triad:** Early-onset hypertension + Hypokalemia + Metabolic alkalosis. * **Key Diagnostic Marker:** Low Renin and Low Aldosterone (distinguishes it from Conn’s syndrome). * **Treatment:** It does **not** respond to Spironolactone (since the defect is distal to the aldosterone receptor). It is treated with ENaC blockers like **Amiloride** or **Triamterene**.

Question 138: What is the normal plasma osmolality in mOsmol/kg?

A. 240-255
B. 260-275
C. 285-295 (Correct Answer)
D. 300-312

Explanation: **Explanation:** The correct answer is **C (285–295 mOsmol/kg)**. Plasma osmolality is a measure of the concentration of solutes (primarily sodium, chloride, bicarbonate, glucose, and urea) in the blood. In a healthy individual, the body maintains this value within a narrow range of **285–295 mOsmol/kg** to ensure cellular stability. This homeostasis is primarily regulated by the **hypothalamic-pituitary-renal axis** through the action of Antidiuretic Hormone (ADH) and the thirst mechanism. **Why the other options are incorrect:** * **Options A & B (240–275):** These values represent **hypo-osmolality**. Such levels are seen in states of water excess or significant hyponatremia (e.g., SIADH), which can lead to cerebral edema as water shifts into cells. * **Option D (300–312):** These values represent **hyper-osmolality**. This occurs in dehydration, diabetes insipidus, or hyperglycemia. A plasma osmolality above 300 mOsmol/kg is a potent stimulus for ADH release and the sensation of thirst. **High-Yield Facts for NEET-PG:** 1. **Calculated Osmolality Formula:** $2[Na^+] + \frac{\text{Glucose}}{18} + \frac{BUN}{2.8}$. Sodium is the primary determinant of plasma osmolality. 2. **Osmolar Gap:** The difference between measured and calculated osmolality. A gap $>10$ suggests the presence of unmeasured osmoles (e.g., Ethanol, Methanol, Ethylene glycol). 3. **Osmoreceptors:** Located in the **OVLT** (Organum Vasculosum of the Lamina Terminalis) and the **SFO** (Subfornical Organ) of the hypothalamus; they sense changes as small as 1%.

Question 139: Which of the following substances relaxes mesangial cells in the glomerulus?

A. Dopamine (Correct Answer)
B. Histamine
C. Angiotensin III
D. Platelet-derived growth factor (PDGF)

Explanation: **Explanation:** The glomerular mesangial cells are specialized contractile cells located between the capillary loops. Their primary function is to regulate the **Glomerular Filtration Rate (GFR)** by altering the available surface area for filtration. When these cells contract, the surface area decreases (reducing GFR); when they relax, the surface area increases (raising GFR). **1. Why Dopamine is Correct:** Dopamine acts on **D1 receptors** in the kidney to cause vasodilation and **relaxation of mesangial cells**. This relaxation increases the effective filtration surface area, contributing to the increase in GFR and natriuresis (sodium excretion) typically seen with low-to-moderate dose dopamine infusions. Other substances that cause relaxation include **Atrial Natriuretic Peptide (ANP)**, cAMP, and Prostaglandin E2 (PGE2). **2. Why the Other Options are Incorrect:** * **Histamine:** Acts as a potent **contractor** of mesangial cells, thereby reducing the filtration surface area. * **Angiotensin II & III:** These are powerful vasoconstrictors. Angiotensin II is the primary hormonal regulator that causes mesangial **contraction** via AT1 receptors. Angiotensin III has similar, though less potent, effects. * **Platelet-derived growth factor (PDGF):** This is a mitogen that stimulates mesangial cell proliferation and **contraction**. It plays a significant role in the pathophysiology of many glomerular diseases (e.g., glomerulonephritis). **High-Yield Clinical Pearls for NEET-PG:** * **Contraction (Decreases GFR):** Angiotensin II, Vasopressin (ADH), Endothelin, Histamine, Noradrenaline, and PAF. * **Relaxation (Increases GFR):** Dopamine, ANP, PGE2, and cAMP. * **Mesangial Function:** They also provide structural support to capillaries and possess phagocytic properties to remove macromolecules from the basement membrane.

Question 140: Which of the following is NOT a component of the countercurrent multiplier mechanism?

A. Vasa recta (Correct Answer)
B. Thick ascending limb of the loop of Henle
C. Thin descending limb of the loop of Henle
D. Collecting duct

Explanation: The renal medullary osmotic gradient is maintained by two distinct systems: the **Countercurrent Multiplier** and the **Countercurrent Exchanger**. ### Why "Vasa Recta" is the Correct Answer The **Vasa recta** functions as a **Countercurrent Exchanger**, not a multiplier. Its role is passive; it prevents the dissipation of the medullary osmotic gradient by removing excess water and solutes without washing away the hypertonicity. It maintains the gradient created by the loop of Henle. ### Analysis of Other Options (The Multipliers) The Countercurrent Multiplier is an active process occurring in the **Loop of Henle** and the **Collecting Duct**: * **Thick Ascending Limb (TAL):** The "engine" of the multiplier. It actively pumps Na⁺, K⁺, and Cl⁻ into the interstitium via the NKCC2 transporter but is impermeable to water. * **Thin Descending Limb:** This segment is highly permeable to water but impermeable to solutes. Water leaves the tubule due to the high interstitial osmolarity created by the TAL. * **Collecting Duct:** Contributes to the multiplier through **Urea Recycling**. Under the influence of ADH, urea moves into the medullary interstitium, accounting for nearly 50% of the hypertonicity. ### High-Yield NEET-PG Pearls * **The "Single Effect":** This refers to the active transport of NaCl out of the TAL, which creates a 200 mOsm/L gradient between the tubule and the interstitium at any given level. * **NKCC2 Transporter:** Targeted by **Loop Diuretics** (e.g., Furosemide), which abolish the medullary gradient and result in dilute urine. * **Vasa Recta Blood Flow:** It is characteristically slow (only ~1-2% of total renal blood flow) to ensure the medullary gradient is not "washed out."

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

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Tubular Reabsorption and Secretion

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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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Integrative Responses to Fluid Challenges

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