Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 291: Ethanol increases urine output by which of the following mechanisms?

A. Inhibiting Na+ reabsorption
B. Suppressing ADH secretion (Correct Answer)
C. Increasing glomerular filtration rate
D. Suppressing H+ secretion

Explanation: ### Explanation **Mechanism of Action** The correct answer is **B. Suppressing ADH secretion**. Ethanol (alcohol) acts as a potent diuretic primarily by inhibiting the release of **Antidiuretic Hormone (ADH)**, also known as Vasopressin, from the posterior pituitary gland. Under normal conditions, ADH acts on the V2 receptors of the collecting ducts to insert **aquaporin-2 channels**, allowing water reabsorption. By suppressing ADH, ethanol prevents these channels from being inserted, leading to decreased water permeability in the distal nephron. This results in the excretion of a large volume of dilute urine (**water diuresis**). **Analysis of Incorrect Options** * **A. Inhibiting Na+ reabsorption:** This is the mechanism of action for diuretics like Furosemide or Thiazides (saluretics). Ethanol does not primarily interfere with sodium transport. * **C. Increasing glomerular filtration rate (GFR):** While minor changes in GFR can occur due to systemic effects, it is not the primary or consistent mechanism behind ethanol-induced diuresis. * **D. Suppressing H+ secretion:** This would affect acid-base balance (potentially causing acidosis) but does not directly lead to increased urine volume. **High-Yield Clinical Pearls for NEET-PG** * **Site of Action:** Ethanol acts centrally on the **supraoptic and paraventricular nuclei** of the hypothalamus. * **The "Breaking the Seal" Phenomenon:** The rapid onset of diuresis after alcohol consumption is due to the immediate drop in plasma ADH levels. * **Dehydration:** The diuresis caused by ethanol is often out of proportion to the fluid intake, leading to the cellular dehydration characteristic of a "hangover." * **Other ADH Inhibitors:** Cold exposure and ANP (Atrial Natriuretic Peptide) also inhibit ADH secretion.

Question 292: Which of the following causes metabolic alkalosis?

A. Diarrhea
B. Chronic renal failure
C. Hyperaldosteronism (Correct Answer)
D. Salicylate poisoning

Explanation: **Explanation:** **Correct Option: C. Hyperaldosteronism** Aldosterone acts on the principal cells of the collecting duct to reabsorb sodium and secrete potassium. Crucially, it also stimulates the **$\alpha$-intercalated cells** to secrete hydrogen ions ($H^+$) via the $H^+$-ATPase pump. In hyperaldosteronism (e.g., Conn’s Syndrome), excessive $H^+$ secretion into the urine leads to a gain of bicarbonate in the blood, resulting in **metabolic alkalosis**. This is typically accompanied by hypokalemia and hypertension. **Incorrect Options:** * **A. Diarrhea:** Causes loss of bicarbonate-rich intestinal secretions, leading to **Normal Anion Gap Metabolic Acidosis (NAGMA)**. * **B. Chronic Renal Failure:** The kidneys fail to excrete fixed acids (phosphates/sulfates) and cannot regenerate sufficient bicarbonate, leading to **High Anion Gap Metabolic Acidosis (HAGMA)**. * **D. Salicylate Poisoning:** Classically causes a mixed acid-base disorder: **Respiratory Alkalosis** (due to direct stimulation of the medullary respiratory center) and **High Anion Gap Metabolic Acidosis** (due to interference with the Krebs cycle and accumulation of organic acids). **High-Yield Clinical Pearls for NEET-PG:** * **Metabolic Alkalosis "Mnemonic":** Remember **"Vomiting & Volume depletion"** and **"Hyperaldosteronism"** as the primary causes. * **Paradoxical Aciduria:** In states of metabolic alkalosis with hypokalemia (like hyperaldosteronism), the kidney excretes $H^+$ instead of $K^+$ to conserve potassium, making the urine acidic despite systemic alkalosis. * **Salicylate Poisoning:** In children, metabolic acidosis often dominates; in adults, a mixed picture is more common.

Question 293: A patient presents with pO2 of 85 mmHg, pCO2 of 50 mmHg, pH of 7.2, and HCO3- of 36 mmol/L. What is the acid-base condition?

A. Respiratory acidosis with compensatory metabolic alkalosis (Correct Answer)
B. Uncompensated respiratory acidosis
C. Respiratory alkalosis with compensatory metabolic acidosis
D. Respiratory acidosis with compensatory metabolic acidosis

Explanation: ### Explanation To solve acid-base questions, follow a systematic three-step approach: **1. Analyze the pH:** The normal pH range is 7.35–7.45. A pH of **7.20** indicates **acidemia**. **2. Identify the Primary Cause:** * **pCO2:** The normal range is 35–45 mmHg. Here, pCO2 is **50 mmHg** (elevated). High CO2 causes acidosis (Respiratory Acidosis). * **HCO3-:** The normal range is 22–28 mmol/L. Here, HCO3- is **36 mmol/L** (elevated). High bicarbonate causes alkalosis. Since the pH is acidic, the primary driver must be the elevated pCO2. Thus, the primary condition is **Respiratory Acidosis**. **3. Determine Compensation:** The body attempts to neutralize the acidosis by increasing HCO3- (Metabolic Alkalosis). Since the HCO3- is significantly elevated (36 mmol/L) and the pH is moving back toward normal (but not yet fully normal), it is **Respiratory acidosis with compensatory metabolic alkalosis**. #### Analysis of Incorrect Options: * **Option B:** In "uncompensated" acidosis, the HCO3- would remain within the normal range (22–28 mmol/L). * **Option C:** Respiratory alkalosis would require a high pH (>7.45) and a low pCO2 (<35 mmHg). * **Option D:** Metabolic acidosis would involve a low HCO3- (<22 mmol/L), which would worsen the acidemia rather than compensate for it. #### NEET-PG High-Yield Pearls: * **ROME Mnemonic:** **R**espiratory **O**pposite (pH ↓, pCO2 ↑), **M**etabolic **E**qual (pH ↑, HCO3- ↑). * **Compensation Rule:** The body never "over-compensates." If the pH is <7.40, the primary process is acidosis; if >7.40, it is alkalosis. * **Renal Compensation:** Unlike respiratory compensation (which takes minutes), renal compensation (altering HCO3- reabsorption) takes **24–72 hours** to reach maximal effect.

Question 294: What is the most potent stimulus for renin release?

A. Sympathetic stimulation
B. Decreased sodium chloride in the distal convoluted tubule
C. Prostacyclin
D. Hypotension (Correct Answer)

Explanation: **Explanation:** Renin release is the rate-limiting step of the Renin-Angiotensin-Aldosterone System (RAAS). While multiple factors stimulate the juxtaglomerular (JG) cells, **hypotension** (specifically a decrease in renal perfusion pressure) is considered the most potent and direct stimulus. 1. **Why Hypotension is correct:** The JG cells in the afferent arteriole act as **intrarenal baroreceptors**. A drop in mean arterial pressure directly stretches these cells less, triggering an immediate and robust release of renin to restore systemic blood pressure and maintain glomerular filtration rate (GFR). 2. **Analysis of Incorrect Options:** * **Sympathetic stimulation (Option A):** Activated via $\beta_1$-adrenergic receptors on JG cells during stress or exercise. While significant, it is often a secondary response to systemic hypotension. * **Decreased NaCl in the DCT (Option B):** This is sensed by the **Macula Densa**. When NaCl delivery drops (indicating low GFR), the macula densa signals the JG cells to release renin. This is a vital feedback mechanism but is generally considered less "potent" than direct baroreceptor activation. * **Prostacyclin (Option C):** Prostaglandins ($PGE_2$ and $PGI_2$) act as local paracrine signaling molecules that stimulate renin release, but they usually act as mediators for the macula densa pathway rather than primary stimuli. **Clinical Pearls for NEET-PG:** * **Location:** Renin is synthesized and stored in the **modified smooth muscle cells** (JG cells) of the **afferent arteriole**. * **Inhibitors:** Renin release is inhibited by **Atrial Natriuretic Peptide (ANP)** and high levels of Angiotensin II (negative feedback). * **Mnemonic:** Stimuli for Renin = **"The 3 Bs"** — **B**aroreceptors (Pressure), **B**ath (NaCl content at Macula Densa), and **B**eta-1 receptors (Sympathetic).

Question 295: What is the glomerular filtration rate (GFR) in a normal person under resting conditions?

A. 125 ml/min (Correct Answer)
B. 90 ml/min
C. 60 ml/min
D. 150 ml/min

Explanation: **Explanation:** The **Glomerular Filtration Rate (GFR)** is defined as the total volume of fluid filtered from the glomerular capillaries into the Bowman’s capsule per unit time. In a healthy adult male of average size (70 kg), the standard GFR is **125 ml/min** (approximately 180 Liters per day). **Why Option A is Correct:** Under resting conditions, the kidneys receive about 20-25% of the cardiac output (Renal Blood Flow ≈ 1100 ml/min). Of the plasma flowing through the glomerulus (Renal Plasma Flow ≈ 625 ml/min), approximately 20% is filtered. This "Filtration Fraction" (GFR/RPF) results in the classic value of **125 ml/min**. **Why Other Options are Incorrect:** * **Option B (90 ml/min):** While 90 ml/min is often considered the lower limit of "normal" in clinical practice, it is not the standard physiological baseline for a healthy young adult. A GFR below 90 ml/min may indicate Stage 1 or 2 Chronic Kidney Disease (CKD) if other markers of kidney damage are present. * **Option C (60 ml/min):** This is a critical clinical threshold. A GFR consistently below 60 ml/min for more than 3 months is the diagnostic criterion for Chronic Kidney Disease (Stage 3). * **Option D (150 ml/min):** This value represents hyperfiltration, which can occur in early stages of diabetic nephropathy or during pregnancy, but it is not the standard resting value. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Marker:** Inulin clearance is the gold standard for measuring GFR because it is freely filtered but neither reabsorbed nor secreted. * **Clinical Marker:** Creatinine clearance is the most common clinical method used to estimate GFR. * **Filtration Fraction:** Normal is 0.2 (20%). * **Daily Urine Output:** Despite filtering 180 L/day, 99% is reabsorbed, resulting in a normal urine output of ~1.5 L/day.

Question 296: Filtration fraction can be best determined by which pair of substances?

A. Inulin and mannitol
B. PAH and Inulin (Correct Answer)
C. Inulin and creatinine
D. PAH and mannitol

Explanation: **Explanation:** **Filtration Fraction (FF)** is the ratio of the Glomerular Filtration Rate (GFR) to the Renal Plasma Flow (RPF). It represents the fraction of plasma entering the kidneys that is actually filtered across the glomerular capillaries into the Bowman’s space. The formula is: **FF = GFR / RPF** 1. **Why Option B is Correct:** * **Inulin** is the gold standard for measuring **GFR** because it is freely filtered but neither reabsorbed nor secreted by the renal tubules. * **Para-aminohippuric acid (PAH)** is used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and actively secreted, resulting in almost complete clearance from the plasma in a single pass through the kidney. * Therefore, the ratio of Inulin clearance to PAH clearance provides the most accurate determination of the Filtration Fraction. 2. **Why Other Options are Incorrect:** * **Options A & D (Mannitol):** While mannitol is used to measure GFR (similar to inulin), it does not measure RPF. * **Option C (Creatinine):** Creatinine is used to estimate GFR in clinical practice, but it is slightly secreted by tubules, making it less accurate than inulin. Using two GFR markers (Inulin and Creatinine) cannot determine the FF as the RPF component is missing. **High-Yield Facts for NEET-PG:** * **Normal FF:** Approximately **20%** (0.20). * **Clinical Significance:** FF increases in conditions like **Congestive Heart Failure** and **Renal Artery Stenosis** (due to a disproportionate decrease in RPF compared to GFR, maintained by Angiotensin II-mediated efferent vasoconstriction). * **Inulin Clearance = GFR; PAH Clearance = ERPF.** * **Creatinine Clearance** slightly overestimates GFR (due to tubular secretion).

Question 297: Which of the following is NOT a constituent of the juxtaglomerular complex?

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

Explanation: The **Juxtaglomerular Apparatus (JGA)** is a specialized structural unit located at the vascular pole of the renal corpuscle. Its primary function is to regulate blood pressure and the glomerular filtration rate (GFR) via the Renin-Angiotensin-Aldosterone System (RAAS) and tubuloglomerular feedback. ### Why "Glomerulus" is the Correct Answer The **Glomerulus** is a network of capillaries where filtration occurs; while the JGA is physically adjacent to it, the glomerulus itself is a distinct part of the renal corpuscle and not a functional component of the JGA complex. ### Explanation of JGA Components (Incorrect Options) * **Macula Densa (Option A):** These are specialized columnar epithelial cells in the **thick ascending limb** of the Loop of Henle (at the transition to the distal tubule). They act as **chemoreceptors** sensing sodium chloride (NaCl) levels in the tubular fluid. * **Juxtaglomerular (JG) Cells (Option C):** These are modified smooth muscle cells located primarily in the wall of the **afferent arteriole**. They act as **baroreceptors** and are responsible for the synthesis, storage, and secretion of **Renin**. * **Extraglomerular Mesangial Cells (Option D):** Also known as **Lacis cells** or Polkissen cells, they are located in the triangular space between the afferent and efferent arterioles. They facilitate communication between the macula densa and JG cells. ### High-Yield NEET-PG Pearls * **Tubuloglomerular Feedback:** When NaCl levels rise (sensed by Macula Densa), the JGA causes constriction of the afferent arteriole to decrease GFR. * **Renin Release:** Stimulated by: 1. Decreased renal perfusion pressure (baroreceptors), 2. Decreased NaCl delivery to macula densa, 3. Sympathetic stimulation ($\beta_1$ receptors). * **Histology:** JG cells contain pro-renin and renin granules, which stain positive with PAS (Periodic Acid-Schiff).

Question 298: Which substance undergoes both filtration and reabsorption in the nephron?

A. Insulin
B. Glucose (Correct Answer)
C. Creatinine
D. Para-aminohippuric acid

Explanation: **Explanation:** The correct answer is **Glucose**. In a healthy individual, glucose is freely filtered at the glomerulus because of its small molecular size. Subsequently, it undergoes **100% reabsorption** in the **Proximal Convoluted Tubule (PCT)** via the SGLT-2 (Sodium-Glucose Linked Cotransporter) and SGLT-1 transporters. Under normal physiological conditions, no glucose is excreted in the urine. **Analysis of Incorrect Options:** * **Inulin (Option A):** Inulin is the "gold standard" for measuring GFR because it is **freely filtered** but is **neither reabsorbed nor secreted** by the renal tubules. (Note: The option says "Insulin," which is a hormone; however, in renal physiology contexts, "Inulin" is the classic distractor). * **Creatinine (Option C):** Creatinine is freely filtered and is **not reabsorbed**. It undergoes a small amount of tubular secretion, which is why creatinine clearance slightly overestimates the actual GFR. * **Para-aminohippuric acid (PAH) (Option D):** PAH is freely filtered and **extensively secreted** by the PCT. It is used to measure **Renal Plasma Flow (RPF)** because it is almost completely cleared from the blood in a single pass through the kidney. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Threshold for Glucose:** Glucose starts appearing in the urine (glycosuria) when blood glucose levels exceed **180 mg/dL**. * **Transport Maximum ($T_m$):** The $T_m$ for glucose is approximately **375 mg/min** in men and **300 mg/min** in women. * **SGLT-2 Inhibitors:** Drugs like Dapagliflozin inhibit glucose reabsorption in the PCT and are used in managing Diabetes Mellitus.

Question 299: All of the following cause renal vasoconstriction except?

A. Angiotensin II
B. Carbon monoxide (Correct Answer)
C. Norepinephrine
D. Endothelin

Explanation: **Explanation:** The regulation of renal blood flow (RBF) and glomerular filtration rate (GFR) depends on the balance between vasoconstrictors and vasodilators. **Why Carbon Monoxide (CO) is correct:** Carbon monoxide acts as a potent **vasodilator** in the renal vasculature. It functions similarly to Nitric Oxide (NO) by stimulating the enzyme **soluble guanylyl cyclase**, which increases intracellular **cGMP** levels. This leads to smooth muscle relaxation and increased renal blood flow. Additionally, CO inhibits the release of endothelin, further promoting vasodilation. **Why the other options are incorrect:** * **Angiotensin II (Option A):** A powerful vasoconstrictor that preferentially constricts the **efferent arterioles** to maintain GFR when renal perfusion pressure is low. * **Norepinephrine (Option B):** Released via sympathetic stimulation, it acts on **$\alpha_1$ adrenoceptors** to cause significant constriction of both afferent and efferent arterioles, reducing RBF. * **Endothelin (Option D):** One of the most potent endogenous vasoconstrictors produced by damaged or stressed endothelial cells. It plays a role in pathological states like acute renal failure. **High-Yield Clinical Pearls for NEET-PG:** * **Vasodilators of the Kidney:** Prostaglandins ($PGE_2$, $PGI_2$), Nitric Oxide (NO), Bradykinin, Atrial Natriuretic Peptide (ANP), Dopamine (at low doses), and Carbon Monoxide. * **Vasoconstrictors of the Kidney:** Angiotensin II, Norepinephrine, Epinephrine, Endothelin, and ADH (Vasopressin). * **Prostaglandin Role:** They act as protective vasodilators during states of high vasoconstriction. This is why **NSAIDs** (which inhibit prostaglandins) can cause acute kidney injury by allowing unopposed vasoconstriction.

Question 300: Oliguria is defined as urine output:

A. <0.5 ml/hr (Correct Answer)
B. 10 ml/hr
C. 20 ml/hr
D. 40 ml/hr

Explanation: **Explanation:** **Oliguria** is clinically defined as a urine output that is insufficient to maintain homeostasis and excrete metabolic waste products. In the context of acute clinical monitoring (especially in ICU or post-operative settings), the standard definition is a urine output of **<0.5 ml/kg/hr** for at least 6 consecutive hours. 1. **Why Option A is Correct:** The value **<0.5 ml/hr** (often simplified from <0.5 ml/kg/hr in exams) is the threshold used by the **KDIGO** and **RIFLE** criteria to define Stage 1 Acute Kidney Injury (AKI). For an average 70 kg adult, this equates to approximately <35 ml/hr or <400–500 ml/day. Below this level, the kidneys cannot effectively clear nitrogenous wastes like urea and creatinine. 2. **Why Other Options are Incorrect:** * **Options B (10 ml/hr) and C (20 ml/hr):** These values fall well below the 0.5 ml/kg/hr threshold for a standard adult. While they represent severe oliguria, they are not the defining "cutoff" point for the diagnosis. * **Option D (40 ml/hr):** For a 70 kg adult, 40 ml/hr is approximately 0.57 ml/kg/hr, which is considered **normal** (though borderline) urine output. **High-Yield Clinical Pearls for NEET-PG:** * **Anuria:** Defined as urine output **<100 ml/day** or <0.1 ml/kg/hr. It is often associated with complete urinary tract obstruction or cortical necrosis. * **Polyuria:** Urine output **>3 Liters/day** or >3 ml/kg/hr. * **Azotemia:** The biochemical hallmark of oliguria, characterized by an elevation of BUN and Serum Creatinine. * **Most Common Cause:** Prerenal azotemia (due to decreased renal perfusion/dehydration) is the most common cause of oliguria encountered in clinical practice.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

Tubular Reabsorption and Secretion

Practice Questions

Concentration and Dilution of Urine

Practice Questions

Acid-Base Regulation by the Kidneys

Practice Questions

Sodium and Water Balance

Practice Questions

Potassium Regulation

Practice Questions

Calcium and Phosphate Handling

Practice Questions

Micturition Physiology

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Renal Function Tests

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Integrative Responses to Fluid Challenges

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