Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 191: Erythropoietin is secreted from which cells?

A. Juxtaglomerular cells
B. Macula densa
C. Interstitial cells (Correct Answer)
D. Glomerulus

Explanation: **Explanation:** **Correct Answer: C. Interstitial cells** Erythropoietin (EPO) is a glycoprotein hormone essential for erythropoiesis. In adults, approximately **90% of EPO** is produced by the **peritubular interstitial cells (fibroblast-like cells)** located in the renal cortex and outer medulla. These cells act as oxygen sensors; when they detect hypoxia (via Hypoxia-Inducible Factor - HIF), they increase EPO production to stimulate red blood cell synthesis in the bone marrow. (Note: In the fetus, the liver is the primary source). **Analysis of Incorrect Options:** * **A. Juxtaglomerular (JG) cells:** These are modified smooth muscle cells in the afferent arteriole. Their primary function is the secretion of **Renin** in response to low blood pressure or sympathetic stimulation, not EPO. * **B. Macula densa:** These are specialized cells in the distal convoluted tubule that sense **sodium chloride (NaCl) concentration**. They regulate GFR via tubuloglomerular feedback and influence renin release. * **D. Glomerulus:** This is the filtration unit of the kidney consisting of capillary loops and podocytes. While it is crucial for ultrafiltration, it does not have an endocrine role in EPO secretion. **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Kidney Disease (CKD):** The destruction of renal interstitium leads to EPO deficiency, resulting in **normocytic normochromic anemia**, which is treated with recombinant human EPO. * **Stimulus:** The primary stimulus for EPO release is **hypoxia**, not the number of RBCs themselves. * **Tumor Association:** Renal Cell Carcinoma (RCC) can cause paraneoplastic **polycythemia** due to ectopic EPO production.

Question 192: What is the maximum osmolarity of urine in the presence of ADH?

A. 50 mOsmol / L
B. 800 mOsmol / L
C. 1200 mOsmol / L (Correct Answer)
D. 500 mOsmol / L

Explanation: ### Explanation **1. Why the Correct Answer is Right (C: 1200 mOsmol/L)** The maximum concentrating ability of the human kidney is approximately **1200 to 1400 mOsmol/L**. This process is governed by **Antidiuretic Hormone (ADH)**, also known as Vasopressin. * **Mechanism:** ADH acts on the V2 receptors of the late distal tubule and collecting ducts, inserting **Aquaporin-2 (AQP2)** channels. * **Countercurrent Multiplier:** The loop of Henle creates a hypertonic medullary interstitium (reaching 1200 mOsmol/L at the tip of the papilla). * **Equilibration:** In the presence of ADH, water is reabsorbed from the collecting duct into the hypertonic medulla until the tubular fluid reaches osmotic equilibrium with the surrounding interstitium. Thus, the final urine osmolarity matches the maximum medullary osmolarity. **2. Why the Other Options are Wrong** * **Option A (50 mOsmol/L):** This represents the **minimum** osmolarity of urine (most dilute) achieved in the absence of ADH (e.g., Diabetes Insipidus or excessive water intake). * **Option B (800 mOsmol/L):** While this is hypertonic compared to plasma (290 mOsmol/L), it does not represent the physiological maximum limit of the human kidney. * **Option D (500 mOsmol/L):** This is a moderately concentrated urine but is far below the maximum capacity of the medullary gradient. **3. NEET-PG High-Yield Pearls** * **Obligatory Urine Volume:** To excrete a daily solute load of 600 mOsmol, a human must produce at least **0.5 L/day** of urine ($600 \div 1200 = 0.5$). * **Urea Recycling:** Urea contributes nearly **50%** of the hypertonicity of the renal medullary interstitium. * **Vasa Recta:** Acts as a **countercurrent exchanger**, maintaining the gradient without washing it out. * **ADH Source:** Synthesized in the **Supraoptic nucleus** (primarily) and Paraventricular nucleus of the hypothalamus; stored in the posterior pituitary.

Question 193: Which of the following substances is primarily filtered with little secretion or reabsorption in the renal tubules?

A. Sodium
B. Creatinine (Correct Answer)
C. Glucose
D. Amino acids

Explanation: **Explanation:** The correct answer is **Creatinine**. This question tests the fundamental concept of how the kidney handles different solutes based on filtration, reabsorption, and secretion. **1. Why Creatinine is Correct:** Creatinine is an endogenous breakdown product of creatine phosphate in muscles. In the nephron, it is **freely filtered** by the glomerulus and undergoes **minimal tubular secretion** (about 10–20%) with **zero reabsorption**. Because the amount excreted in urine is nearly equal to the amount filtered, it serves as a practical clinical marker for estimating the Glomerular Filtration Rate (GFR). **2. Why the Other Options are Incorrect:** * **Sodium (A):** Sodium is freely filtered but extensively **reabsorbed** (approx. 99%) throughout the nephron (Proximal Tubule, Loop of Henle, and Distal Tubule) to maintain fluid and electrolyte balance. * **Glucose (C) and Amino Acids (D):** These are essential nutrients. They are freely filtered but are **completely reabsorbed** (100%) in the Proximal Convoluted Tubule (PCT) via secondary active transport (SGLT transporters for glucose). They should not appear in the urine under normal physiological conditions. **Clinical Pearls for NEET-PG:** * **Inulin:** The "Gold Standard" for measuring GFR because it is freely filtered and undergoes **neither** secretion nor reabsorption. * **Creatinine Clearance:** Slightly **overestimates** GFR because of the small amount of tubular secretion. * **Para-aminohippuric acid (PAH):** Used to measure **Renal Plasma Flow (RPF)** because it is filtered and then almost entirely secreted, resulting in near-total clearance from the blood in a single pass. * **Transport Maximum ($T_m$):** Glucose begins to appear in urine (glycosuria) when blood glucose exceeds the renal threshold of approximately **180 mg/dL**.

Question 194: What is true about the function of Angiotensin II?

A. Constriction of the afferent arteriole
B. Autoregulation of GFR
C. Secretion from the endothelium
D. All of the above (Correct Answer)

Explanation: **Explanation:** Angiotensin II (AT-II) is a potent vasoconstrictor and a central regulator of renal hemodynamics. The correct answer is **All of the above** because AT-II performs multiple integrated functions to maintain renal stability: 1. **Constriction of Arterioles (Option A):** While AT-II preferentially constricts the **efferent arteriole** (to maintain GFR during low perfusion), it also causes constriction of the **afferent arteriole**, especially at higher physiological concentrations. This global vasoconstriction helps increase systemic blood pressure. 2. **Autoregulation of GFR (Option B):** AT-II is a key mediator of the tubuloglomerular feedback and the renin-angiotensin-aldosterone system (RAAS). By modulating the resistance of both afferent and efferent arterioles, it ensures that the Glomerular Filtration Rate (GFR) remains relatively constant despite fluctuations in systemic arterial pressure. 3. **Secretion/Production (Option C):** While the primary conversion of Angiotensin I to II occurs via ACE in the pulmonary capillaries, the **renal vascular endothelium** also contains ACE. Furthermore, the kidney has a local intra-renal RAAS where AT-II is produced locally to act in a paracrine fashion. **High-Yield Clinical Pearls for NEET-PG:** * **Preferential Action:** At low concentrations, AT-II primarily constricts the **efferent arteriole**. This increases glomerular hydrostatic pressure to prevent GFR from dropping during hypotension. * **ACE Inhibitors/ARBs:** These drugs block AT-II action, leading to efferent vasodilation. This can cause a slight "dip" in GFR but is protective in diabetic nephropathy by reducing hyperfiltration. * **Other Actions:** AT-II also stimulates **Aldosterone** release (adrenal cortex) and **ADH** release (posterior pituitary), and directly increases proximal tubule Na+/H+ exchange.

Question 195: Erythropoietin is secreted mainly by:

A. Kidney (Correct Answer)
B. Liver
C. Brain
D. RBC

Explanation: **Explanation:** **1. Why the Kidney is Correct:** Erythropoietin (EPO) is a glycoprotein hormone essential for erythropoiesis. In adults, approximately **85–90%** of EPO is synthesized and secreted by the **peritubular interstitial fibroblasts** (specifically the extraglomerular mesangial cells and cells in the renal cortex/outer medulla). These cells act as oxygen sensors; when renal tissue hypoxia occurs (detected via HIF-1α), EPO production increases to stimulate the bone marrow to produce more red blood cells. **2. Why Other Options are Incorrect:** * **Liver:** The liver is the primary source of EPO during **fetal life**. In adults, it contributes only about **10–15%** of total EPO production (primarily via hepatocytes and Ito cells). It cannot compensate sufficiently if the kidneys fail. * **Brain:** While small amounts of EPO are produced in the brain (astrocytes) to act as a neuroprotective agent, it does not contribute to systemic erythropoiesis. * **RBC:** Red blood cells are the *target* of EPO action (specifically the erythroid progenitor cells in the bone marrow), not the site of production. **3. Clinical Pearls for NEET-PG:** * **Chronic Kidney Disease (CKD):** The most common cause of anemia in CKD is the deficiency of EPO due to the destruction of peritubular interstitial cells. * **Stimulus:** The primary stimulus for EPO release is **hypoxia** (low $PO_2$), not anemia itself. * **Polycythemia:** Certain tumors, such as **Renal Cell Carcinoma (RCC)** and Hepatocellular Carcinoma, can secrete ectopic EPO, leading to secondary polycythemia. * **Recombinant EPO:** Used clinically to treat anemia in CKD patients and those undergoing chemotherapy.

Question 196: Which of the following statements is true regarding oliguria?

A. Urine formation less than 50 mL/ 24 hours
B. Urine formation less than 250 mL/ 24 hours
C. Urine formation of less than 500 mL/24 hours (Correct Answer)
D. Urine formation less than 750 mL/24 hours

Explanation: ### Explanation **Concept of Obligatory Urine Volume** The correct answer is **C (Urine formation of less than 500 mL/24 hours)**. This definition is rooted in the concept of **obligatory urine volume**. To maintain homeostasis, an average adult must excrete approximately 600 mOsm of metabolic waste (solutes) daily. Since the maximum concentrating capacity of the human kidney is about 1200 mOsm/L, the minimum volume of water required to flush these solutes out is: * $600\text{ mOsm} \div 1200\text{ mOsm/L} = 0.5\text{ L (or 500 mL)}$ Therefore, any volume below 500 mL/day is insufficient to clear metabolic waste, leading to **azotemia** (elevation of nitrogenous products). **Analysis of Incorrect Options:** * **Option A (< 50 mL/day):** This defines **Anuria**. Clinically, anuria is often defined as <100 mL/day, but <50 mL is the strict threshold used to indicate total suppression of urine or complete urinary tract obstruction. * **Option B (< 250 mL/day):** This is a sub-threshold of oliguria but does not represent the standard clinical definition. * **Option D (< 750 mL/day):** While this is a decrease from the average 1.5 L/day output, it is still sufficient to clear daily solute loads and does not meet the criteria for oliguria. **High-Yield Clinical Pearls for NEET-PG:** * **Polyuria:** Urine output > 3 L/day (seen in Diabetes Mellitus and Diabetes Insipidus). * **Normal Urine Output:** 0.5 to 2.0 mL/kg/hour. * **Pediatric Oliguria:** Defined as < 0.5 mL/kg/hour in children or < 1.0 mL/kg/hour in infants. * **Prerenal vs. Renal:** In prerenal oliguria, the **Fractional Excretion of Sodium (FeNa)** is typically < 1%, whereas in Acute Tubular Necrosis (Intrinsic Renal), it is > 2%.

Question 197: Which cells are responsible for acid secretion in the kidney?

A. I cells (Correct Answer)
B. P cells
C. Mesangial cells
D. Pericytes

Explanation: **Explanation:** The correct answer is **A. I cells (Intercalated cells)**. In the renal physiology of the distal convoluted tubule and collecting duct, two distinct cell types exist: Principal (P) cells and Intercalated (I) cells. **Type A Intercalated cells** are specifically responsible for **acid secretion**. They utilize apical H⁺-ATPase and H⁺/K⁺-ATPase pumps to secrete hydrogen ions into the tubular lumen while reabsorbing bicarbonate (HCO₃⁻) via basolateral Cl⁻/HCO₃⁻ exchangers. This process is vital for maintaining systemic acid-base balance. (Note: Type B Intercalated cells perform the opposite function, secreting bicarbonate in states of alkalosis). **Analysis of Incorrect Options:** * **B. P cells (Principal cells):** These are the most abundant cells in the collecting duct. Their primary role is **sodium reabsorption** (via ENaC channels) and **potassium secretion**, regulated largely by Aldosterone and ADH. * **C. Mesangial cells:** Located within the glomerulus, these cells provide structural support to glomerular capillaries, possess contractile properties to regulate the glomerular filtration rate (GFR), and have phagocytic functions. * **D. Pericytes:** In the kidney, these are perivascular cells located around the vasa recta. They play a role in regulating medullary blood flow and are the primary source of **Erythropoietin (EPO)** production in response to hypoxia. **High-Yield Clinical Pearls for NEET-PG:** * **Distal Renal Tubular Acidosis (Type 1 RTA):** Caused by a functional defect in the **Type A Intercalated cells**, leading to an inability to acidify urine (urine pH > 5.5). * **Aldosterone** acts on P cells to increase Na⁺ reabsorption and on I cells to stimulate H⁺ secretion. * **Carbonic Anhydrase II** is present in I cells to generate H⁺ and HCO₃⁻ from CO₂ and H₂O.

Question 198: Non-oliguric renal failure is caused by damage to which part of the nephron?

A. Afferent arteriole
B. Efferent arteriole
C. Ascending limb of the Loop of Henle
D. Collecting duct (Correct Answer)

Explanation: **Explanation:** **1. Why the Collecting Duct is Correct:** Non-oliguric renal failure (NORF) is a clinical state where the kidneys fail to concentrate urine despite a decrease in the Glomerular Filtration Rate (GFR). The **Collecting Duct (CD)** is the primary site for final urine concentration under the influence of Antidiuretic Hormone (ADH). Damage to the collecting duct cells (often due to nephrotoxins like aminoglycosides or partial obstruction) results in a loss of responsiveness to ADH. Consequently, the kidney cannot reabsorb water, leading to a high output of dilute urine (polyuria or normal volume) despite failing excretory functions. **2. Why the Other Options are Incorrect:** * **Afferent & Efferent Arterioles (A & B):** Damage to these vessels primarily affects the GFR and renal blood flow. Significant damage here usually leads to **oliguric** renal failure because the initial filtration pressure drops sharply, reducing urine formation at the source. * **Ascending Limb of the Loop of Henle (C):** While this segment is crucial for the countercurrent multiplier system, damage here typically presents as specific electrolyte wasting (like Bartter-like syndromes). While it affects concentration, the definitive "non-oliguric" failure pattern is most classically associated with distal tubular and collecting duct dysfunction. **3. NEET-PG High-Yield Pearls:** * **Definition:** Non-oliguric renal failure is defined as acute kidney injury (AKI) with a urine output **>400 ml/day**. * **Common Causes:** Aminoglycoside toxicity (e.g., Gentamicin), Amphotericin B, and contrast-induced nephropathy are classic triggers for non-oliguric AKI. * **Prognosis:** Non-oliguric AKI generally has a **better prognosis** and lower mortality rate compared to oliguric AKI. * **Key Concept:** In NORF, the "quality" of filtration is lost (waste retention) even if the "quantity" of urine remains normal.

Question 199: Which of the following is not a part of the glomerular filtration barrier?

A. Mesangial cell (Correct Answer)
B. Endothelial cell
C. Podocyte
D. Basement membrane

Explanation: The **glomerular filtration barrier (GFB)** is a highly specialized three-layered structure responsible for the ultrafiltration of blood while preventing the passage of plasma proteins. ### Why Mesangial Cells are the Correct Answer **Mesangial cells** are located between the glomerular capillaries but are **not** part of the filtration interface itself. Their primary functions include providing structural support to the capillary loops, secreting the extracellular matrix, and performing phagocytosis to remove trapped macromolecules. While they can influence the glomerular filtration rate (GFR) by contracting in response to Angiotensin II (thereby reducing surface area), they do not form a layer of the barrier through which filtrate must pass. ### Analysis of Incorrect Options (Components of the GFB) 1. **Endothelial cells (Option B):** The innermost layer. These are **fenestrated** capillaries (pores of 70–100 nm) that prevent the passage of blood cells but allow most plasma components through. 2. **Basement membrane (Option D):** The middle layer. It is composed of Type IV collagen and heparan sulfate proteoglycans. It acts as a **size and charge barrier** (negatively charged), repelling proteins like albumin. 3. **Podocytes (Option C):** The outermost layer (visceral epithelium). They possess foot processes (pedicels) that interdigitate to form **filtration slits** bridged by a diaphragm containing the protein **nephrin**. ### High-Yield Clinical Pearls for NEET-PG * **Charge Selectivity:** The GFB is negatively charged due to sialoglycoproteins and heparan sulfate. Loss of this negative charge (e.g., in **Minimal Change Disease**) leads to selective albuminuria. * **Nephrin:** Mutations in the gene encoding nephrin (*NPHS1*) result in **Finnish-type congenital nephrotic syndrome**. * **Goodpasture Syndrome:** Characterized by antibodies against the alpha-3 chain of Type IV collagen in the glomerular basement membrane.

Question 200: Which of the following ions is least absorbed in the renal tubules?

A. Sodium
B. Urea
C. Creatinine (Correct Answer)
D. Glucose

Explanation: **Explanation:** The renal handling of substances depends on the balance between filtration, reabsorption, and secretion. The correct answer is **Creatinine** because it is the only substance among the options that undergoes **zero reabsorption** in the renal tubules. **1. Why Creatinine is Correct:** Creatinine is a metabolic byproduct of muscle metabolism. Once filtered at the glomerulus, it is **not reabsorbed** by the tubules. In fact, a small amount is actively secreted into the lumen. Because the amount excreted in urine is slightly more than the amount filtered, creatinine clearance is used as a clinical marker to estimate the Glomerular Filtration Rate (GFR). **2. Why the Other Options are Incorrect:** * **Sodium (A):** Sodium is the most actively reabsorbed ion. Approximately **99%** of filtered sodium is reabsorbed throughout the nephron (65% in the proximal tubule) to maintain fluid and electrolyte balance. * **Urea (B):** Urea is a waste product, but it is still partially reabsorbed. About **40-50%** of filtered urea is reabsorbed passively, primarily in the proximal tubule and medullary collecting ducts (where it contributes to the medullary osmotic gradient). * **Glucose (D):** Under normal physiological conditions, **100%** of filtered glucose is reabsorbed in the Proximal Convoluted Tubule (PCT) via SGLT2 and SGLT1 transporters. It only appears in urine if the blood glucose level exceeds the renal threshold (~180 mg/dL). **High-Yield Clinical Pearls for NEET-PG:** * **Inulin:** The "gold standard" for GFR measurement because it is freely filtered and undergoes **neither reabsorption nor secretion**. * **PAH (Para-aminohippurate):** Used to measure Renal Plasma Flow (RPF) because it is filtered and almost entirely secreted, resulting in near-total clearance. * **Renal Threshold for Glucose:** 180 mg/dL; **Transport Maximum (TmG):** 375 mg/min in men, 300 mg/min in women.

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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