Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 421: Which substance is not significantly absorbed in the loop of Henle?

A. K+
B. Urea (Correct Answer)
C. Cl-
D. Na+

Explanation: **Explanation:** The Loop of Henle (LoH) plays a critical role in the countercurrent multiplier system, primarily focusing on the reabsorption of electrolytes to create a hypertonic medullary interstitium. **Why Urea is the correct answer:** Urea is unique because it is primarily **secreted** into the thin descending limb of the Loop of Henle rather than being absorbed. While urea is heavily reabsorbed in the Proximal Convoluted Tubule (50%) and the Medullary Collecting Ducts (via UT-A1/A3 transporters under ADH influence), the Loop of Henle itself is not a site of significant urea absorption. Instead, urea "recycles" from the medullary interstitium back into the tubular lumen of the LoH to maintain the osmotic gradient. **Why the other options are incorrect:** * **Na+, Cl-, and K+:** These electrolytes are significantly reabsorbed in the **Thick Ascending Limb (TAL)** of the Loop of Henle. This occurs via the **NKCC2 transporter** (Sodium-Potassium-2 Chloride symporter). Approximately 25% of the filtered load of these ions is reclaimed here. This segment is known as the "diluting segment" because it is impermeable to water but highly active in solute reabsorption. **High-Yield Clinical Pearls for NEET-PG:** * **NKCC2 Transporter:** This is the target of **Loop Diuretics** (e.g., Furosemide). Inhibition leads to powerful diuresis and potential hypokalemia. * **Bartter Syndrome:** A genetic defect in the NKCC2 transporter (or related channels in the TAL) that mimics chronic loop diuretic use. * **Countercurrent Multiplier:** The TAL reabsorbs NaCl without water, while the Descending Limb is permeable to water but not solutes. This separation is essential for concentrating urine. * **Urea Recycling:** This process is essential for maintaining the high osmolarity of the inner medulla, which allows for water reabsorption in the collecting ducts.

Question 422: Hypertonic urine is excreted due to water absorption in which part of the nephron?

A. Collecting ducts (Correct Answer)
B. Distal convoluted tubule (DCT)
C. Ascending limb of the loop of Henle
D. Descending limb of the loop of Henle

Explanation: ### Explanation The final concentration of urine (hypertonicity) is primarily determined in the **collecting ducts** under the influence of **Antidiuretic Hormone (ADH/Vasopressin)**. **1. Why Option A is Correct:** While water is reabsorbed in various segments of the nephron, the collecting duct is the **final site** where the urine concentration is adjusted. In the presence of ADH, **Aquaporin-2 channels** are inserted into the apical membrane of the collecting duct cells. This allows water to move out of the tubule into the highly medullary interstitium via osmosis, resulting in the excretion of concentrated (hypertonic) urine. **2. Why the Other Options are Incorrect:** * **Distal Convoluted Tubule (DCT):** The early DCT is part of the "diluting segment." It is relatively impermeable to water but actively reabsorbs solutes, making the tubular fluid more dilute (hypotonic), not hypertonic. * **Ascending Limb of the Loop of Henle:** This segment is **impermeable to water** but actively transports NaCl into the interstitium (via the NKCC2 transporter). It is known as the "diluting segment" because it removes solute without water, decreasing tubular tonicity. * **Descending Limb of the Loop of Henle:** While water is reabsorbed here, this segment contributes to the **Countercurrent Multiplier** system to create a medullary gradient. It does not determine the final tonicity of the excreted urine. **Clinical Pearls for NEET-PG:** * **V2 Receptors:** ADH acts on V2 receptors in the collecting duct to increase cAMP, leading to Aquaporin-2 insertion. * **Obligatory Water Reabsorption:** Occurs in the PCT (65%) and is independent of ADH. * **Facultative Water Reabsorption:** Occurs in the collecting ducts and is **ADH-dependent**. * **Diabetes Insipidus:** A deficiency of ADH (Central) or resistance to it (Nephrogenic) prevents water reabsorption in the collecting ducts, leading to large volumes of dilute (hypotonic) urine.

Question 423: Which of the following statements regarding nephron function is true?

A. The ascending thick limb is permeable to water.
B. The descending thin limb is impermeable to water.
C. The osmolality of intra-tubular content in the distal convoluted tubule (DCT) is greater than the surrounding interstitium.
D. The osmolality of intratubular content in the proximal convoluted tubule (PCT) is isotonic to the surrounding interstitium. (Correct Answer)

Explanation: ### Explanation **Correct Option: D** In the **Proximal Convoluted Tubule (PCT)**, approximately 65-70% of filtered water and solutes (like Na+, K+, and glucose) are reabsorbed. Because the PCT is highly permeable to water (via Aquaporin-1 channels), water follows the reabsorbed solutes osmotically in a 1:1 ratio. This process is known as **obligatory water reabsorption**, ensuring that the tubular fluid remains **isotonic** (approximately 300 mOsm/L) to the surrounding plasma and interstitium throughout the length of the PCT. **Analysis of Incorrect Options:** * **Option A:** The **Thick Ascending Limb (TAL)** is known as the "diluting segment." It actively reabsorbs solutes (via the Na-K-2Cl symporter) but is **impermeable to water**. This makes the tubular fluid dilute. * **Option B:** The **Descending Thin Limb** is highly **permeable to water** but relatively impermeable to solutes. As it descends into the hypertonic medulla, water leaves the tubule, concentrating the fluid. * **Option C:** By the time fluid reaches the **Distal Convoluted Tubule (DCT)**, it is **hypotonic** (approx. 100-150 mOsm/L) compared to the interstitium because solutes were removed in the ascending limb without water. **High-Yield Clinical Pearls for NEET-PG:** * **Countercurrent Multiplier:** Established by the Loop of Henle; the ascending limb is the key site for solute removal without water. * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** Act on the PCT to inhibit glucose reabsorption, leading to glucosuria. * **Carbonic Anhydrase Inhibitors (Acetazolamide):** Act primarily in the PCT, the site of maximum bicarbonate reabsorption. * **ADH (Vasopressin):** Acts on the **Collecting Ducts** (not PCT) to regulate "facultative" water reabsorption via Aquaporin-2 channels.

Question 424: What is the normal glomerular filtration rate?

A. 50 ml/min
B. 125 ml/min (Correct Answer)
C. 250 ml/min
D. 500 ml/min

Explanation: **Explanation:** **Glomerular Filtration Rate (GFR)** is the volume of fluid filtered from the renal glomerular capillaries into the Bowman’s capsule per unit of time. In a healthy adult male of average size (1.73 m² body surface area), the normal GFR is approximately **125 ml/min**. This equates to roughly **180 Liters per day**, meaning the entire plasma volume is filtered and processed by the kidneys about 60 times daily. * **Why 125 ml/min is correct:** This value represents the physiological norm. It is determined by the Net Filtration Pressure (NFP) and the Capillary Filtration Coefficient ($K_f$). * **Why other options are incorrect:** * **50 ml/min:** This indicates significant renal impairment (Stage 3 Chronic Kidney Disease). * **250 ml/min & 500 ml/min:** These values are pathologically high and exceed the physiological capacity of the human renal system. **High-Yield Clinical Pearls for NEET-PG:** * **Gender Difference:** GFR is slightly lower in females (approx. 110 ml/min) compared to males. * **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, though it slightly overestimates it due to minor tubular secretion. * **Filtration Fraction:** This is the ratio of GFR to Renal Plasma Flow (RPF). Normal value is ~0.2 (20%). * **Autoregulation:** GFR remains constant between mean arterial pressures of **75 to 160 mmHg** due to myogenic mechanisms and tubuloglomerular feedback.

Question 425: The hyperosmolarity of the renal medulla is due to increased content of what?

A. Potassium (K+)
B. Sodium (Na+) (Correct Answer)
C. Glucose
D. Divalent cations (e.g., Ca2+, Mg2+)

Explanation: ### Explanation The hyperosmolarity of the renal medulla is essential for the kidney's ability to concentrate urine. This medullary osmotic gradient is primarily established and maintained by two substances: **Sodium Chloride (NaCl)** and **Urea**. **1. Why Sodium (Na+) is Correct:** The **Countercurrent Multiplier system**, located in the Loop of Henle, is the primary mechanism for generating this gradient. The Thick Ascending Limb (TAL) actively reabsorbs Sodium, Potassium, and Chloride via the **NKCC2 transporter**. Because the TAL is impermeable to water, these solutes accumulate in the medullary interstitium, significantly increasing its osmolarity. Sodium, being the primary extracellular cation, contributes to approximately half of the medullary hyperosmolarity (the other half being contributed by Urea recycling). **2. Why the other options are incorrect:** * **Potassium (K+):** While K+ is co-transported in the TAL, it is mostly recycled back into the tubular lumen via ROMK channels to maintain the activity of the NKCC2 transporter. It does not accumulate in the interstitium in concentrations high enough to drive the osmotic gradient. * **Glucose:** In a healthy kidney, glucose is entirely reabsorbed in the Proximal Convoluted Tubule (PCT). It does not reach the medulla and therefore plays no role in the medullary osmotic gradient. * **Divalent cations (Ca2+, Mg2+):** These are reabsorbed paracellularly in the TAL, driven by the positive luminal potential, but their interstitial concentrations are too low to significantly impact total medullary osmolarity. **Clinical Pearls for NEET-PG:** * **Vasa Recta:** Acts as a **Countercurrent Exchanger**, maintaining the gradient by removing excess water without washing out the solutes. * **Loop Diuretics (e.g., Furosemide):** These inhibit the NKCC2 transporter, "washing out" the medullary gradient and resulting in the inability to concentrate urine. * **ADH (Vasopressin):** Increases medullary hyperosmolarity by stimulating **Urea transporters (UT-A1)** in the collecting ducts, promoting urea recycling.

Question 426: Which of the following substances is NOT significantly absorbed in the proximal convoluted tubule (PCT)?

A. Bicarbonate
B. Sodium
C. Phosphate
D. H+ (Correct Answer)

Explanation: The **Proximal Convoluted Tubule (PCT)** is the "workhorse" of the nephron, responsible for the bulk reabsorption of essential solutes. ### **Why H+ is the Correct Answer** Hydrogen ions ($H^+$) are **secreted**, not absorbed, in the PCT. This occurs primarily via the **$Na^+$-$H^+$ Exchanger 3 (NHE3)** on the apical membrane. This secretion is a vital step in acid-base balance, as the secreted $H^+$ reacts with filtered bicarbonate ($HCO_3^-$) to facilitate its indirect reabsorption. Therefore, $H^+$ moves from the peritubular capillaries/tubular cells into the lumen. ### **Why the Other Options are Incorrect** * **Bicarbonate ($HCO_3^-$):** Approximately **80-90%** of filtered bicarbonate is reabsorbed in the PCT. This is mediated by carbonic anhydrase (CA), which converts luminal $HCO_3^-$ to $CO_2$ and $H_2O$ for cellular entry. * **Sodium ($Na^+$):** About **65%** of filtered sodium is reabsorbed here via various symporters (with glucose/amino acids) and antiporters. It is the primary driving force for the reabsorption of water and other solutes. * **Phosphate ($PO_4^{3-}$):** Roughly **80%** of filtered phosphate is reabsorbed in the PCT via $Na^+$-$PO_4^{3-}$ cotransporters (NaPi-IIa). This process is clinically significant as it is inhibited by **Parathyroid Hormone (PTH)**. ### **High-Yield NEET-PG Pearls** * **Obligatory Water Reabsorption:** 65% of water is reabsorbed in the PCT iso-osmotically. * **Glucose & Amino Acids:** 100% are reabsorbed in the early PCT (S1 segment). * **Carbonic Anhydrase Inhibitors (Acetazolamide):** Act specifically on the PCT, leading to bicarbonate loss and metabolic acidosis. * **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, urate, and phosphate in the urine.

Question 427: What is the effective renal blood flow in humans?

A. 425 mL/min
B. 525 mL/min
C. 625 mL/min (Correct Answer)
D. 725 mL/min

Explanation: ### Explanation **1. Why Option C is Correct:** The effective renal plasma flow (ERPF) is typically measured using **Para-aminohippuric acid (PAH) clearance**, as PAH is both filtered and secreted, resulting in almost complete extraction from the blood. * **The Calculation:** Total Renal Blood Flow (RBF) in a healthy 70 kg adult is approximately **1100–1200 mL/min** (about 20-25% of cardiac output). * Since the average hematocrit is 45%, the Renal Plasma Flow (RPF) is roughly 55% of the RBF. * $1100 \text{ mL/min} \times 0.55 = 605 \text{ mL/min}$. * In standard medical texts (like Guyton and Ganong), the average value for **Effective Renal Plasma Flow** is cited as **625 mL/min**. **2. Why Other Options are Incorrect:** * **Option A (425 mL/min) & B (525 mL/min):** These values are too low for a healthy adult. Such values would indicate significant renal vasoconstriction or a state of shock/hypovolemia. * **Option D (725 mL/min):** This value is higher than the physiological average for plasma flow. While RBF can increase during pregnancy or high protein intake, 625 mL/min remains the standard "textbook" value for exams. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Filtration Fraction (FF):** Calculated as GFR/RPF. Normal FF is approximately **20%** ($125/625 = 0.2$). * **PAH Clearance:** It underestimates true RPF by about 10% because some blood supplies non-secreting portions of the kidney (like the medulla and perirenal fat). Therefore, **True RPF = ERPF / 0.9**. * **Gold Standard:** While PAH measures ERPF, **Inulin clearance** is the gold standard for measuring GFR. * **Renal Oxygen Consumption:** The kidneys have the highest oxygen consumption per gram of tissue after the heart, primarily to fuel the $Na^+/K^+$ ATPase pump for sodium reabsorption.

Question 428: Which of the following substances is least filtered by the glomerulus?

A. Haemoglobin (Correct Answer)
B. Albumin
C. Myoglobin
D. Inulin

Explanation: ### Explanation The filtration of substances across the glomerular filtration barrier (GFB) depends on two primary factors: **molecular size (radius)** and **electrical charge**. **1. Why Haemoglobin is the Correct Answer:** The GFB consists of the fenestrated endothelium, the basement membrane, and the podocyte slit diaphragms. While **Albumin** (69 kDa) is the classic example of a protein restricted by its negative charge (polyanionic barrier), **Haemoglobin** has a significantly larger molecular weight (~64.5 to 68 kDa in its tetrameric form). However, the crucial factor here is its **effective molecular radius**. Haemoglobin is a large, bulky tetramer that is significantly more restricted than albumin or myoglobin. Under normal physiological conditions, free haemoglobin is also bound to **haptoglobin**, creating a massive complex that is completely non-filterable. **2. Analysis of Incorrect Options:** * **Albumin (B):** Although it has a similar molecular weight to haemoglobin, it is smaller in radius. It is primarily restricted by the **negative charge** of the basement membrane (heparan sulfate). Small amounts are filtered but reabsorbed in the PCT. * **Myoglobin (C):** This is a monomer with a low molecular weight (~17 kDa). It is easily filtered by the glomerulus, which is why rhabdomyolysis leads to myoglobinuria and potential renal toxicity. * **Inulin (D):** A polysaccharide with a molecular weight of ~5 kDa. It is **freely filtered**, neither reabsorbed nor secreted, making it the gold standard for measuring GFR. **3. NEET-PG High-Yield Pearls:** * **Size Cut-off:** Neutral molecules with a radius < 20 Å are freely filtered; those > 42 Å are not filtered. * **Charge Selectivity:** The GFB is negatively charged. Therefore, for the same size, **Cationic > Neutral > Anionic** substances in terms of filterability. * **Minimal Change Disease:** Characterized by the loss of the negative charge (heparan sulfate) on the GFB, leading to massive albuminuria. * **Haemoglobinuria:** Occurs only when intravascular hemolysis exceeds the binding capacity of haptoglobin and the reabsorptive capacity of the tubules.

Question 429: Erythropoietin is inhibited by which hormone?

A. Estrogen (Correct Answer)
B. Progesterone
C. Thyroxine
D. Testosterone

Explanation: **Explanation:** The production of **Erythropoietin (EPO)**, the primary regulator of erythropoiesis, is primarily stimulated by renal hypoxia. However, sex steroids and other hormones significantly modulate its secretion and action. **Why Estrogen is the Correct Answer:** **Estrogen** is known to **inhibit** erythropoiesis. It acts by suppressing the production of erythropoietin in the kidneys and directly inhibiting the proliferation of erythroid stem cells in the bone marrow. This inhibitory effect is one of the physiological reasons why women of reproductive age typically have lower hemoglobin and hematocrit levels compared to men. **Analysis of Incorrect Options:** * **Testosterone (Option D):** This is a potent **stimulator** of erythropoietin. Testosterone increases EPO production from the kidneys and enhances the sensitivity of erythroid progenitors to EPO. This explains the higher hemoglobin levels in males. * **Thyroxine (Option C):** Thyroid hormones **stimulate** erythropoiesis by increasing the metabolic rate and oxygen consumption of tissues, which creates a state of relative hypoxia that triggers EPO release. * **Progesterone (Option B):** Unlike estrogen, progesterone does not have a significant inhibitory effect on erythropoietin; in some contexts, it may even mildly stimulate ventilation, indirectly affecting oxygenation. **High-Yield Clinical Pearls for NEET-PG:** * **Site of EPO Production:** 85% from the **Peritubular interstitial cells** of the renal cortex; 15% from the liver (Kupffer cells/hepatocytes). * **Stimulants of EPO:** Hypoxia (most potent), Testosterone, Thyroxine, ACTH, Catecholamines (via β-receptors), and Prostaglandins (PGE2). * **Inhibitors of EPO:** Estrogen and Chronic Kidney Disease (due to loss of peritubular cells). * **Mechanism:** EPO acts via the **JAK2/STAT pathway** to prevent apoptosis of erythroid precursor cells (CFU-E).

Question 430: Which of the following substances are freely filtered by the kidney across glomerular capillaries?

A. Creatinine, HCO3-, and Glucose (Correct Answer)
B. Albumin, Creatinine, and HCO3-
C. Globulin, HCO3-, and Glucose
D. Albumin, HCO3-, and Glucose

Explanation: ### Explanation The glomerular filtration barrier consists of the fenestrated endothelium, the glomerular basement membrane (GBM), and the podocyte slit diaphragms. Filtration is determined by two primary factors: **molecular size** and **electrical charge**. **1. Why Option A is Correct:** * **Creatinine (113 Da):** A small metabolic waste product that is freely filtered. * **Bicarbonate (HCO3-) (61 Da):** A small ion that passes easily into the Bowman’s space. * **Glucose (180 Da):** A small organic molecule that is freely filtered (and normally 100% reabsorbed in the proximal tubule). All three substances have a molecular weight well below the threshold of **70,000 Daltons** and are not significantly hindered by the negative charge of the basement membrane. **2. Why Other Options are Incorrect:** * **Options B & D (Albumin):** Albumin has a molecular weight of ~69,000 Da. While it is near the size cutoff, it is **negatively charged**. The GBM contains heparan sulfate (polyanionic), which electrostatically repels albumin, preventing its filtration. * **Option C (Globulin):** Globulins are much larger proteins (90,000 to 150,000+ Da) than albumin. Their large size and charge prevent them from crossing the filtration barrier. **High-Yield Clinical Pearls for NEET-PG:** * **Filtration Fraction:** The ratio of GFR to Renal Plasma Flow (Normal ≈ 0.2 or 20%). * **Neutral vs. Charged:** For the same molecular radius, **cationic** (positive) substances are filtered most easily, followed by neutral substances, while **anionic** (negative) substances are filtered the least. * **Minimal Change Disease:** Characterized by the loss of the negative charge on the GBM (loss of polyanions), leading to selective proteinuria (albuminuria) despite no visible structural change on light microscopy.

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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