Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 71: Which hormone exerts short-term regulation of blood pressure through the kidney?

A. Antidiuretic Hormone (ADH) (Correct Answer)
B. Angiotensin II
C. Epinephrine
D. Aldosterone

Explanation: **Explanation:** The regulation of blood pressure by the kidney involves both short-term (neural/hormonal) and long-term (hemodynamic) mechanisms. **Antidiuretic Hormone (ADH)**, also known as Vasopressin, acts as a short-term regulator through two primary mechanisms: 1. **V1 Receptors:** Located on vascular smooth muscle, causing rapid vasoconstriction to increase systemic vascular resistance. 2. **V2 Receptors:** Located in the late distal tubule and collecting ducts, increasing water reabsorption via Aquaporin-2 channels. While its volume effect is more gradual, its potent vasoconstrictive action provides immediate BP support during acute states like hemorrhage. **Analysis of Incorrect Options:** * **Angiotensin II:** While a potent vasoconstrictor, it is part of the Renin-Angiotensin-Aldosterone System (RAAS), which is primarily considered an intermediate to long-term regulator of blood pressure and fluid balance. * **Epinephrine:** Though it acts rapidly on the heart and blood vessels (short-term), it is a catecholamine released from the adrenal medulla, not a hormone that regulates BP specifically *through the kidney*. * **Aldosterone:** This is a classic long-term regulator. It acts via gene transcription to increase sodium and water reabsorption, a process that takes hours to days to significantly impact blood pressure. **NEET-PG High-Yield Pearls:** * **ADH Stimulus:** The most potent stimulus for ADH release is increased plasma osmolarity (detected by osmoreceptors), but the most "urgent" stimulus is a decrease in ECF volume (detected by baroreceptors). * **V1 vs. V2:** Remember **V1 = Vascular** (Constriction) and **V2 = Volume** (Water reabsorption). * **Atrial Natriuretic Peptide (ANP):** The "counter-regulatory" hormone to RAAS; it lowers BP by promoting natriuresis and vasodilation.

Question 72: What is the normal daily urinary protein excretion in a healthy adult?

A. Approximately 100 mg (Correct Answer)
B. Approximately 500 mg
C. Approximately 1 gm
D. Approximately 2 gm

Explanation: **Explanation:** In a healthy adult, the glomerular filtration barrier (composed of fenestrated endothelium, basement membrane, and podocytes) is highly selective [1]. It restricts the passage of large proteins like albumin based on both size and negative charge [1]. Consequently, the normal daily urinary protein excretion is **less than 150 mg/day**, with an average of **approximately 100 mg/day** [1]. Of this total protein: * **40% is Albumin:** Only a tiny fraction (approx. 30 mg) escapes the filter and remains unabsorbed [2]. * **60% is Globulins:** This includes **Tamm-Horsfall protein** (uromodulin), which is secreted by the thick ascending limb of the Loop of Henle and constitutes the bulk of non-albumin protein. **Analysis of Incorrect Options:** * **Option B (500 mg):** This level indicates **pathological proteinuria**. Values between 150 mg and 3.5 g/day are seen in various renal diseases (nephritic range). * **Options C & D (1 gm & 2 gm):** These represent significant proteinuria. Persistent excretion at these levels suggests glomerular damage or tubular dysfunction. Excretion exceeding **3.5 g/day** is the hallmark of **Nephrotic Syndrome**. **High-Yield Clinical Pearls for NEET-PG:** * **Microalbuminuria:** Defined as 30–300 mg/day of albumin. It is the earliest clinical sign of diabetic nephropathy. * **Tamm-Horsfall Protein:** The most abundant protein in normal urine; it forms the matrix of urinary casts. * **Selectivity:** The glomerular basement membrane contains **Heparan Sulfate**, which provides a negative charge that repels albumin [1]. Loss of this charge leads to "minimal change disease."

Question 73: What is the major site of glomerular filtrate reabsorption?

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

Explanation: **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the "workhorse" of the nephron, responsible for the bulk reabsorption of the glomerular filtrate. Approximately **65-70%** of the total filtered water, sodium, and chloride is reabsorbed here. Furthermore, 100% of filtered glucose and amino acids, and about 80-90% of bicarbonate, are reabsorbed in the PCT. This high capacity is due to the presence of a prominent **brush border (microvilli)**, which significantly increases the surface area for transport. **Analysis of Incorrect Options:** * **Loop of Henle:** This segment is primarily involved in the countercurrent multiplier system to concentrate urine. It reabsorbs about 15-25% of filtered sodium but is not the major site of overall volume reabsorption. * **Distal Convoluted Tubule (DCT):** This is a site for "fine-tuning" electrolyte balance (reabsorbing ~5% of sodium). It is relatively impermeable to water unless acted upon by hormones. * **Collecting Duct:** This is the final site for water reabsorption (regulated by ADH) and acid-base balance. While critical for determining final urine concentration, it handles only a small fraction (about 5-10%) of the initial filtrate. **High-Yield Clinical Pearls for NEET-PG:** * **Obligatory Water Reabsorption:** Occurs in the PCT and descending limb of the Loop of Henle; it is independent of ADH. * **Isotonic Reabsorption:** The fluid remaining at the end of the PCT is **isotonic** to plasma because water follows solutes proportionately. * **Fanconi Syndrome:** A clinical condition resulting from generalized dysfunction of the PCT, leading to the loss of glucose, amino acids, and phosphates in the urine.

Question 74: Insulin clearance typically measures:

A. 135 ml/min
B. 90 ml/min
C. 625 ml/min
D. 125 ml/min (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. 125 ml/min**. **Underlying Concept:** Inulin (a fructose polymer) is the "gold standard" for measuring the **Glomerular Filtration Rate (GFR)**. This is because inulin is freely filtered by the glomeruli and is neither reabsorbed nor secreted by the renal tubules. Therefore, the amount of inulin excreted in the urine per unit time is exactly equal to the amount filtered. In a healthy adult, the average GFR is approximately **125 ml/min** (or 180 L/day). **Analysis of Incorrect Options:** * **A (135 ml/min):** While GFR can vary slightly by body surface area and gender (often cited as 125 ± 15 ml/min), 125 ml/min is the standard physiological value used in examinations. * **B (90 ml/min):** This value is lower than the normal GFR. In clinical practice, a GFR below 90 ml/min/1.73m² is often used as the threshold for Stage 2 Chronic Kidney Disease (CKD). * **C (625 ml/min):** This value represents the **Effective Renal Plasma Flow (ERPF)**, typically measured using Para-aminohippuric acid (PAH) clearance. PAH is both filtered and secreted, making its clearance much higher than inulin. **High-Yield Clinical Pearls for NEET-PG:** * **Creatinine Clearance:** In clinical settings, endogenous creatinine is used to estimate GFR. However, it slightly **overestimates** GFR (by ~10-20%) because a small amount of creatinine is secreted by the tubules. * **Filtration Fraction (FF):** Calculated as GFR / Renal Plasma Flow (125 / 625). The normal FF is **0.2 (or 20%)**. * **Criteria for GFR Marker:** To measure GFR, a substance must be non-toxic, not metabolized by the kidney, and physiologically inert. Inulin fits all these criteria perfectly.

Question 75: What is the clearance of a substance when its concentration in the plasma is 10 mg/dL, its concentration in urine is 100 mg/dL, and the urine flow rate is 2 mL/min?

A. 2 mL/min
B. 10 mL/min
C. 20 mL/min (Correct Answer)
D. 200 mL/min

Explanation: ### Explanation **Concept and Calculation:** Renal clearance refers to the volume of plasma from which a substance is completely removed by the kidneys per unit of time. It is a fundamental measure of renal function. The formula for calculating clearance ($C$) is: $$C = \frac{U \times V}{P}$$ Where: * **$U$** = Concentration of the substance in urine ($100\text{ mg/dL}$) * **$V$** = Urine flow rate ($2\text{ mL/min}$) * **$P$** = Concentration of the substance in plasma ($10\text{ mg/dL}$) Plugging in the values: $$C = \frac{100 \times 2}{10} = \frac{200}{10} = \mathbf{20\text{ mL/min}}$$ **Analysis of Options:** * **Option C (Correct):** Correctly applies the formula, yielding $20\text{ mL/min}$. * **Option A (2 mL/min):** This is simply the urine flow rate ($V$). It ignores the concentration gradient between plasma and urine. * **Option B (10 mL/min):** This represents the U/P ratio ($100/10$) without multiplying by the flow rate. * **Option D (200 mL/min):** This is the product of $U \times V$ (the excretion rate) without dividing by the plasma concentration. **High-Yield Clinical Pearls for NEET-PG:** 1. **Inulin Clearance:** Inulin is the "gold standard" for measuring **GFR** because it is freely filtered but neither reabsorbed nor secreted. 2. **Creatinine Clearance:** Used clinically to estimate GFR; it slightly **overestimates** GFR because a small amount of creatinine is secreted by the tubules. 3. **PAH Clearance:** Para-aminohippuric acid (PAH) is used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and almost completely secreted. 4. **Clearance Ratio:** If $C_x / C_{\text{inulin}} < 1$, the substance undergoes net reabsorption (e.g., Glucose). If $> 1$, it undergoes net secretion.

Question 76: What causes relaxation of mesangial cells in the kidney?

A. cAMP (Correct Answer)
B. Endothelin
C. PGF2
D. Vasopressin

Explanation: **Explanation:** Mesangial cells are specialized smooth muscle-like cells located within the renal glomerulus. Their primary function is to regulate the glomerular filtration rate (GFR) by altering the surface area available for filtration. Like vascular smooth muscle, their contraction and relaxation are governed by intracellular signaling pathways. **1. Why cAMP is correct:** Relaxation of mesangial cells is mediated by agents that increase intracellular **cAMP** (cyclic Adenosine Monophosphate) or **cGMP**. Increased cAMP levels lead to a decrease in intracellular calcium and the inhibition of myosin light chain kinase, resulting in cell relaxation. This increases the effective filtration surface area and GFR. Key physiological relaxants include **Atrial Natriuretic Peptide (ANP)**, **Dopamine**, and **PGE2**. **2. Why the other options are incorrect:** * **Endothelin (B):** One of the most potent vasoconstrictors; it increases intracellular calcium, leading to mesangial contraction and a decrease in GFR. * **PGF2 (C):** Prostaglandin F2-alpha acts as a contractile agent for mesangial cells, unlike PGE2 which causes relaxation. * **Vasopressin (D):** Also known as ADH, it acts via V1 receptors on mesangial cells to cause contraction, thereby reducing the surface area for filtration. **High-Yield Clinical Pearls for NEET-PG:** * **Contractile Agents (Decrease GFR):** Angiotensin II (most important), Vasopressin, Endothelin, Noradrenaline, Platelet-activating factor (PAF), and Thromboxane A2. * **Relaxant Agents (Increase GFR):** ANP, cAMP, Dopamine, PGE2, and Nitric Oxide (NO). * **Function:** Mesangial cells also provide structural support to capillaries and possess phagocytic properties to remove macromolecules from the glomerular basement membrane.

Question 77: Which transporter is present in the ascending loop of Henle?

A. Na+ Cl contransporter
B. Na+ - 2Cl - K+ cotransporter (Correct Answer)
C. ENaC channel
D. Na+ amino acid contransporter

Explanation: ### Explanation **Correct Answer: B. Na+ - 2Cl - K+ cotransporter (NKCC2)** The **Thick Ascending Limb (TAL)** of the Loop of Henle is known as the "diluting segment" of the nephron. The primary transporter responsible for solute reabsorption here is the **NKCC2** (sodium-potassium-2-chloride cotransporter). This symporter moves one Na⁺, one K⁺, and two Cl⁻ ions from the tubular lumen into the epithelial cell, driven by the sodium gradient created by the basolateral Na⁺/K⁺-ATPase. Because the TAL is impermeable to water, the reabsorption of these solutes decreases the osmolarity of the tubular fluid, making it dilute. **Analysis of Incorrect Options:** * **A. Na+ Cl- cotransporter (NCC):** This transporter is located in the **Distal Convoluted Tubule (DCT)**. It is the target of Thiazide diuretics. * **C. ENaC channel:** Epithelial Sodium Channels (ENaC) are found in the **Principal cells of the Collecting Duct**. They are regulated by Aldosterone and inhibited by Amiloride. * **D. Na+ amino acid cotransporter:** These are primarily located in the **Proximal Convoluted Tubule (PCT)**, where the bulk of nutrient reabsorption (glucose, amino acids) occurs. **High-Yield Clinical Pearls for NEET-PG:** * **Loop Diuretics:** Drugs like Furosemide and Bumetanide work by inhibiting the NKCC2 transporter in the TAL. * **Bartter Syndrome:** A genetic defect in the NKCC2 transporter (or associated channels in the TAL) mimics the effect of chronic loop diuretic use, presenting with hypokalemia, metabolic alkalosis, and hypercalciuria. * **Lumen-Positive Potential:** Some K⁺ leaks back into the lumen via ROMK channels, creating a positive charge that drives the paracellular reabsorption of **Calcium and Magnesium** in the TAL. This is why loop diuretics can lead to hypocalcemia/hypomagnesemia.

Question 78: Relaxation of mesangial cells is responsible for which of the following functions?

A. Absorption of glucose
B. Maintenance of blood flow (Correct Answer)
C. Osmotic gradient in medulla
D. Secretion of uric acid

Explanation: **Explanation:** **Mesangial cells** are specialized pericytes located within the renal corpuscle. They possess contractile properties similar to smooth muscle cells, containing actin and myosin filaments. Their primary physiological role is the **regulation of the glomerular filtration rate (GFR)** by altering the surface area available for filtration. 1. **Why Option B is Correct:** When mesangial cells **relax**, the glomerular capillaries expand, increasing the effective surface area for filtration. Conversely, when they contract (stimulated by Angiotensin II or ADH), the surface area decreases, reducing GFR. By modulating the resistance and surface area of the glomerular tuft, mesangial cells play a critical role in the **maintenance and regulation of glomerular blood flow** and filtration dynamics. 2. **Why Other Options are Incorrect:** * **Option A (Glucose Absorption):** This occurs exclusively in the **Proximal Convoluted Tubule (PCT)** via SGLT-2 and SGLT-1 transporters, not in the glomerulus. * **Option C (Osmotic Gradient):** The medullary osmotic gradient is maintained by the **Loop of Henle** (countercurrent multiplier) and the **Vasa Recta** (countercurrent exchanger). * **Option D (Uric Acid Secretion):** This is a tubular function occurring primarily in the PCT via organic anion transporters. **High-Yield Clinical Pearls for NEET-PG:** * **Contraction Stimuli:** Angiotensin II, Vasopressin (ADH), Endothelin, and Histamine cause mesangial contraction (decreasing GFR). * **Relaxation Stimuli:** ANP (Atrial Natriuretic Peptide), Dopamine, and cAMP-inducing agents cause mesangial relaxation (increasing GFR). * **Other Functions:** Mesangial cells also provide structural support to capillary loops and possess phagocytic properties to remove immune complexes (relevant in glomerulonephritis).

Question 79: The proximal convoluted tubule (PCT) absorbs all except:

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

Explanation: The **Proximal Convoluted Tubule (PCT)** is the "workhorse" of the nephron, responsible for the bulk reabsorption of essential solutes. ### 1. Why Hydrogen ions (D) is the correct answer: The PCT is primarily a site of **secretion** for Hydrogen ions ($H^+$), not absorption. This occurs via the **$Na^+$-$H^+$ exchanger (NHE3)** on the apical membrane. While the PCT reabsorbs ~80% of filtered bicarbonate ($HCO_3^-$), it does so by secreting $H^+$ into the lumen to react with bicarbonate. Therefore, $H^+$ is moving out of the blood/cell and into the tubular fluid. ### 2. Why the other options are incorrect: * **Sodium (A):** Approximately **65%** of filtered Sodium is reabsorbed in the PCT through both active transport (Na+/K+ ATPase) and passive pathways. It is the driving force for most other solutes. * **Amino acids (B):** Under normal physiological conditions, **100%** of filtered amino acids are reabsorbed in the early PCT via secondary active transport with Sodium. * **Glucose (C):** Like amino acids, **100%** of filtered glucose is reabsorbed in the early PCT (via SGLT2 and SGLT1 transporters) unless the renal threshold (~180 mg/dL) is exceeded. ### 3. High-Yield Clinical Pearls for NEET-PG: * **Obligatory Water Reabsorption:** The PCT is the only segment where water reabsorption is "obligatory" (follows solutes isosmotically), regardless of ADH levels. * **Fanconi Syndrome:** A generalized dysfunction of the PCT leading to the loss of glucose, amino acids, uric acid, and phosphate in the urine. * **Carbonic Anhydrase Inhibitors (Acetazolamide):** These drugs act specifically on the PCT to inhibit $HCO_3^-$ reabsorption and $H^+$ secretion, leading to alkaline urine. * **SGLT2 Inhibitors (Dapagliflozin):** Modern diabetic drugs that act on the PCT to induce glucosuria.

Question 80: Juxtaglomerular cells secrete which hormone?

A. Angiotensin
B. Renin (Correct Answer)
C. Atrial natriuretic peptide
D. Erythropoietin

Explanation: **Explanation:** The **Juxtaglomerular (JG) cells** are specialized smooth muscle cells located primarily in the wall of the **afferent arteriole** at the point where it contacts the distal convoluted tubule. These cells act as intrarenal baroreceptors that sense changes in renal perfusion pressure. When blood pressure drops or sympathetic stimulation increases, JG cells undergo degranulation to release **Renin** into the bloodstream. Renin is the rate-limiting enzyme of the Renin-Angiotensin-Aldosterone System (RAAS), responsible for converting Angiotensinogen to Angiotensin I. **Analysis of Incorrect Options:** * **A. Angiotensin:** This is a peptide hormone. Angiotensin I is formed in the plasma, and Angiotensin II is primarily formed in the **lungs** via the action of Angiotensin-Converting Enzyme (ACE). * **C. Atrial Natriuretic Peptide (ANP):** This hormone is secreted by the **atrial myocytes** of the heart in response to atrial stretch (fluid overload). It acts as an antagonist to the RAAS. * **D. Erythropoietin (EPO):** While also produced in the kidney, EPO is secreted by **interstitial peritubular cells** (fibroblasts) in the renal cortex, usually in response to hypoxia. **High-Yield Clinical Pearls for NEET-PG:** * **The Juxtaglomerular Apparatus (JGA)** consists of three components: JG cells (secrete renin), Macula Densa (sodium sensors), and Lacis cells (extraglomerular mesangial cells). * **Stimuli for Renin Release:** 1. Decreased renal perfusion pressure (baroreceptor mechanism), 2. Decreased NaCl delivery to Macula Densa, 3. Sympathetic stimulation (via **$\beta_1$ receptors**). * **Bartter Syndrome:** Characterized by hyperplasia of the JG apparatus, leading to hyperreninemia and hypokalemic metabolic alkalosis.

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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