Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 351: In renal glycosuria, what is the renal threshold for glucose?

A. Low (Correct Answer)
B. High
C. Same
D. Greatly increased

Explanation: **Explanation:** In a healthy individual, glucose is freely filtered at the glomerulus and almost entirely reabsorbed in the proximal convoluted tubule (PCT) via SGLT2 and SGLT1 transporters. The **Renal Threshold** is the plasma glucose concentration at which the transporters become saturated, and glucose begins to appear in the urine (glycosuria). In normal adults, this threshold is approximately **180 mg/dL**. **1. Why "Low" is correct:** **Renal Glycosuria** (also known as Benign Glycosuria) is a condition where glucose is excreted in the urine despite **normal** blood glucose levels. This occurs because of a functional defect in the SGLT2 transporters or a reduced affinity for glucose. Because the "spillover" point into the urine happens at a much lower plasma concentration than normal (e.g., 120 mg/dL instead of 180 mg/dL), the renal threshold is considered **Low**. **2. Why other options are wrong:** * **High / Greatly Increased:** A high threshold would mean the kidney is "better" at retaining glucose, which occurs in conditions like chronic kidney disease (CKD) or aging, where the GFR decreases. * **Same:** If the threshold were the same, glycosuria would only occur during hyperglycemia (Diabetes Mellitus), not in renal glycosuria. **High-Yield Clinical Pearls for NEET-PG:** * **Transport Maximum ($T_m$):** The maximum rate of glucose reabsorption, normally **375 mg/min** in men and **303 mg/min** in women. * **Splay:** The difference between the actual renal threshold and the theoretical $T_m$ (due to nephron heterogeneity). * **SGLT2 Inhibitors (e.g., Dapagliflozin):** These drugs pharmacologically **lower the renal threshold** to treat Diabetes Mellitus by inducing glycosuria. * **Fanconi Syndrome:** A generalized PCT defect where a low renal threshold for glucose is seen alongside aminoaciduria, phosphaturia, and bicarbonaturia.

Question 352: What is the effect of sympathetic stimulation on Glomerular Filtration Rate (GFR)?

A. Increased GFR with afferent arteriolar dilatation
B. Increased GFR with efferent arteriolar constriction
C. Decreased GFR with afferent arteriolar constriction (Correct Answer)
D. Decreased GFR with efferent arteriolar dilatation

Explanation: ### Explanation **1. Why Option C is Correct:** The sympathetic nervous system exerts its effect on the kidneys primarily through the release of norepinephrine, which acts on **$\alpha_1$-adrenergic receptors**. While these receptors are present on both arterioles, they are more densely populated on the **afferent arterioles**. Strong sympathetic stimulation (as seen in hemorrhage, exercise, or stress) causes significant **vasoconstriction of the afferent arteriole**. This increases resistance and reduces the hydrostatic pressure within the glomerular capillaries ($P_{GC}$). Since $P_{GC}$ is the primary driving force for filtration, its reduction leads to a **decrease in GFR**. This mechanism serves to divert blood flow away from the kidneys toward vital organs like the heart and brain during emergencies. **2. Why Other Options are Incorrect:** * **Option A:** Sympathetic stimulation causes vasoconstriction, not dilatation. Dilatation would increase GFR, which is the opposite of the physiological response to stress. * **Option B:** While mild sympathetic stimulation can constrict the efferent arteriole (which might transiently maintain GFR), strong stimulation always results in dominant afferent constriction, leading to an overall decrease in GFR. * **Option D:** Sympathetic activity does not cause efferent dilatation. Efferent dilatation would lower $P_{GC}$ and GFR, but it is not the mechanism of sympathetic action. **3. Clinical Pearls & High-Yield Facts:** * **Autoregulation:** At rest, renal autoregulation (Myogenic and Tubuloglomerular feedback) overrides mild sympathetic tone to keep GFR constant. * **Renin Release:** Sympathetic stimulation also acts on **$\beta_1$ receptors** on juxtaglomerular cells to increase **Renin secretion**, activating the RAAS pathway. * **Goldblatt Kidney:** This concept relates to renal artery stenosis, where decreased perfusion mimics chronic sympathetic-like constriction, leading to secondary hypertension. * **Key Formula:** $GFR = K_f \times [(P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS})]$. Sympathetic activity primarily reduces $P_{GC}$.

Question 353: A hypertensive patient is found to have a partial obstruction of the renal artery due to an atherosclerotic plaque. The resultant decrease in blood flow causes increased release of an enzyme from which of the following structures?

A. Afferent arterioles
B. Arcuate arteries
C. Juxtaglomerular cells (Correct Answer)
D. Kupffer cells

Explanation: **Explanation:** The correct answer is **C. Juxtaglomerular cells.** **Mechanism:** This scenario describes **Renovascular Hypertension**. A partial obstruction of the renal artery (e.g., due to atherosclerosis) leads to a decrease in renal perfusion pressure. This drop in pressure is sensed by the **intrarenal baroreceptors** located in the **Juxtaglomerular (JG) cells**, which are specialized modified smooth muscle cells found primarily in the walls of the **afferent arterioles**. In response to decreased stretch (hypotension) or sympathetic stimulation, these cells secrete the enzyme **Renin**. Renin initiates the Renin-Angiotensin-Aldosterone System (RAAS), leading to systemic vasoconstriction and sodium retention to restore blood pressure. **Analysis of Incorrect Options:** * **A. Afferent arterioles:** While JG cells are located *within* the walls of the afferent arterioles, the specific structure responsible for enzyme (Renin) secretion is the JG cell itself. In NEET-PG, always choose the most specific anatomical structure provided. * **B. Arcuate arteries:** These are conduit vessels located at the corticomedullary junction. They do not possess specialized secretory functions for blood pressure regulation. * **D. Kupffer cells:** These are specialized macrophages located in the **liver** sinusoids. They are part of the reticuloendothelial system and are not involved in the RAAS. **High-Yield Clinical Pearls for NEET-PG:** * **Goldblatt Kidney:** This clinical scenario is the human equivalent of the "one-kidney, one-clip" Goldblatt model of hypertension. * **Stimuli for Renin Release:** 1) Decreased perfusion pressure (baroreceptors), 2) Decreased NaCl delivery to the Macula Densa, and 3) Sympathetic stimulation (Beta-1 receptors). * **Histology:** JG cells contain prorenin granules and are considered the "endocrine" component of the Juxtaglomerular Apparatus (JGA).

Question 354: Which aquaporin channel is primarily mediated through ADH action?

A. GLUT
B. Aquaporin 1
C. Aquaporin 2 (Correct Answer)
D. Aquaporin 3

Explanation: **Explanation:** The correct answer is **Aquaporin 2 (AQP2)**. **Why Aquaporin 2 is correct:** Antidiuretic Hormone (ADH), also known as Vasopressin, regulates water reabsorption in the **Principal cells** of the late distal tubule and collecting ducts. When ADH binds to the **V2 receptors** on the basolateral membrane, it triggers a cAMP-mediated signaling pathway. This causes the translocation of intracellular vesicles containing **Aquaporin 2** channels to the **apical (luminal) membrane**. This increases the water permeability of the membrane, allowing water to be reabsorbed into the hypertonic medullary interstitium. **Why other options are incorrect:** * **GLUT:** These are glucose transporters (e.g., GLUT2 in the proximal tubule) and are not involved in water transport. * **Aquaporin 1:** These channels are constitutively active (always open) and are primarily located in the **Proximal Convoluted Tubule (PCT)** and the descending limb of the Loop of Henle. They are **not** regulated by ADH. * **Aquaporin 3 & 4:** These channels are located on the **basolateral membrane** of the collecting duct cells. While they facilitate the exit of water from the cell into the blood, they are constitutively expressed and not the primary site of ADH-mediated regulation. **High-Yield Clinical Pearls for NEET-PG:** * **Diabetes Insipidus (DI):** Central DI is caused by a deficiency of ADH, while Nephrogenic DI is often due to a defect in the V2 receptor or the **AQP2 channel** itself. * **V2 Receptor:** G-protein coupled receptor (Gs) $\rightarrow$ Adenylyl cyclase $\rightarrow$ cAMP $\rightarrow$ Protein Kinase A $\rightarrow$ AQP2 insertion. * **Lithium:** A common cause of drug-induced nephrogenic DI as it inhibits the signaling pathway that leads to AQP2 expression.

Question 355: Prolonged immobilization leads to which of the following conditions?

A. Hypercalcemia (Correct Answer)
B. Hypocalcemia
C. Hyperkalemia
D. Hypokalemia

Explanation: **Explanation:** The correct answer is **Hypercalcemia**. **Mechanism of Action:** The primary driver behind hypercalcemia in prolonged immobilization is the **uncoupling of bone remodeling**. Bone health is dependent on mechanical loading (weight-bearing). When a patient is immobilized (e.g., due to spinal cord injury, extensive casting, or prolonged bed rest), the lack of mechanical stress leads to a significant increase in **osteoclastic bone resorption** and a decrease in osteoblastic bone formation. This rapid breakdown of the bone matrix releases large amounts of calcium into the extracellular fluid, exceeding the kidneys' capacity to excrete it, resulting in hypercalcemia. **Analysis of Incorrect Options:** * **B. Hypocalcemia:** This is incorrect because immobilization triggers bone breakdown, which increases serum calcium levels rather than decreasing them. * **C & D. Hyperkalemia/Hypokalemia:** While prolonged immobilization can lead to muscle atrophy, it does not typically cause significant primary shifts in potassium levels. Potassium imbalances are more commonly associated with acute cell lysis (Rhabdomyolysis) or renal failure, rather than the mechanical unloading of bone. **High-Yield Clinical Pearls for NEET-PG:** * **Hypercalciuria:** This often precedes hypercalcemia in immobilized patients, significantly increasing the risk of **nephrolithiasis** (calcium stones). * **Suppressed PTH:** In immobilization-induced hypercalcemia, the Parathyroid Hormone (PTH) levels will be **low** (suppressed) due to negative feedback from high serum calcium. * **Management:** The treatment of choice for severe immobilization-induced hypercalcemia is **Bisphosphonates**, which inhibit osteoclast activity, alongside aggressive hydration. * **Differential:** Always distinguish this from Primary Hyperparathyroidism, where PTH would be elevated.

Question 356: What is the primary function of I cells in the kidney?

A. Sodium reabsorption
B. Chloride reabsorption
C. Hydrogen ion secretion (Correct Answer)
D. Potassium ion secretion

Explanation: **Explanation:** The **Intercalated cells (I cells)** are specialized cells located in the **late distal tubule and collecting ducts** of the nephron. They play a pivotal role in maintaining the body's acid-base balance. **Why Option C is Correct:** There are two types of I cells: **Type A (Alpha)** and **Type B (Beta)**. * **Type A Intercalated Cells:** These are primarily responsible for **Hydrogen ion (H+) secretion** into the tubular lumen via the H+-ATPase and H+/K+-ATPase pumps. This process is essential for acid excretion and the generation of new bicarbonate (HCO3-) during states of acidosis. * **Type B Intercalated Cells:** These secrete bicarbonate and reabsorb H+ during alkalosis. Since the primary physiological role of I cells (specifically Type A) is managing acid-base status through proton transport, **Hydrogen ion secretion** is the most accurate function. **Why Other Options are Incorrect:** * **Option A & D:** Sodium (Na+) reabsorption and Potassium (K+) secretion are the primary functions of **Principal cells (P cells)**, which are regulated by Aldosterone. * **Option B:** Chloride reabsorption occurs throughout the nephron (mostly via the paracellular pathway or specific symporters like NCC in the DCT), but it is not the defining "primary function" of I cells. **High-Yield Clinical Pearls for NEET-PG:** * **Type 1 Renal Tubular Acidosis (Distal RTA):** Caused by a failure of Type A intercalated cells to secrete H+, leading to a high urinary pH (>5.5) and systemic metabolic acidosis. * **P cells vs. I cells:** Remember **P**rincipal cells handle **P**otassium (secretion), while **I**ntercalated cells handle **I**ons (H+). * **Aldosterone:** While it primarily acts on P cells, it also stimulates H+ secretion in Type A intercalated cells.

Question 357: Renin is secreted by which part of the nephron?

A. Proximal Convoluted Tubule (PCT)
B. Distal Convoluted Tubule (DCT)
C. Collecting Duct
D. Juxtaglomerular Apparatus (Correct Answer)

Explanation: **Explanation:** The correct answer is **Juxtaglomerular Apparatus (JGA)**. Renin is a proteolytic enzyme synthesized, stored, and secreted by the **Juxtaglomerular (JG) cells**, which are specialized modified smooth muscle cells located primarily in the afferent arteriole. These cells form a critical component of the JGA, situated at the point where the thick ascending limb of the Loop of Henle meets the afferent and efferent arterioles of its parent nephron. Renin secretion is the rate-limiting step of the **Renin-Angiotensin-Aldosterone System (RAAS)**, triggered by decreased renal perfusion pressure, reduced sodium delivery to the macula densa, or sympathetic stimulation. **Why other options are incorrect:** * **Proximal Convoluted Tubule (PCT):** The PCT is primarily responsible for the bulk reabsorption of water, electrolytes (65%), glucose, and amino acids. It does not have endocrine secretory functions related to renin. * **Distal Convoluted Tubule (DCT):** While the early part of the DCT contains the **Macula Densa** (which senses NaCl levels and signals the JG cells), the DCT cells themselves do not secrete renin. * **Collecting Duct:** This part of the nephron is involved in the final concentration of urine under the influence of ADH and aldosterone but lacks renin-secreting capabilities. **High-Yield Clinical Pearls for NEET-PG:** * **Stimuli for Renin Release:** 1. Decreased BP (Baroreceptors in afferent arteriole), 2. Decreased NaCl at Macula Densa, 3. Sympathetic activity ($\beta_1$ receptors). * **Inhibitor:** Atrial Natriuretic Peptide (ANP) inhibits renin release. * **Location:** JG cells are located in the **tunica media** of the afferent arteriole. * **Function:** Renin converts Angiotensinogen (from the liver) to Angiotensin I.

Question 358: Glucose and amino acids are reabsorbed from the proximal convoluted tubule or absorbed from the intestinal lumen by which transport mechanism?

A. Symport (Correct Answer)
B. Antiport
C. Uniport
D. Primary active transport

Explanation: ### Explanation The correct answer is **Symport** (also known as Co-transport). **1. Why Symport is Correct:** In both the Proximal Convoluted Tubule (PCT) and the small intestine, glucose and amino acids are transported against their concentration gradients. This is achieved via **Secondary Active Transport**. * **Mechanism:** The **Na⁺-K⁺ ATPase pump** (on the basolateral membrane) creates a steep electrochemical gradient for Sodium (Na⁺) to enter the cell. * **The Process:** Specialized carrier proteins (like **SGLT-1** in the gut and **SGLT-2** in the PCT) use the energy from this Na⁺ gradient to pull glucose/amino acids into the cell. Because both Na⁺ and the nutrient move in the **same direction** across the apical membrane, the mechanism is classified as **Symport**. **2. Why Other Options are Incorrect:** * **Antiport (Counter-transport):** This involves two substances moving in opposite directions (e.g., the Na⁺-H⁺ exchanger in the PCT). * **Uniport:** This involves a single substance moving down its gradient (e.g., **GLUT-2** transporting glucose out of the basolateral membrane into the blood). * **Primary Active Transport:** This refers to transport directly powered by ATP hydrolysis (e.g., Na⁺-K⁺ ATPase). While symport *depends* on this, the actual movement of glucose/amino acids is secondary. **3. High-Yield Clinical Pearls for NEET-PG:** * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A major class of drugs for Diabetes Mellitus that work by inhibiting glucose reabsorption in the PCT, causing glucosuria. * **Hartnup Disease:** A defect in the symport carrier for neutral amino acids (like Tryptophan) in the gut and kidneys. * **Renal Threshold for Glucose:** Approximately **180 mg/dL**. Beyond this plasma concentration, SGLT transporters become saturated ($T_m$ is reached), and glucose appears in the urine.

Question 359: Which substance is used to measure renal perfusion?

A. Inulin
B. PAH (Correct Answer)
C. Creatinine
D. Mannitol

Explanation: **Explanation:** The correct answer is **Para-aminohippuric acid (PAH)**. **Why PAH is the correct answer:** To measure **Renal Plasma Flow (RPF)** and subsequently **Renal Blood Flow (Renal Perfusion)**, a substance must be filtered at the glomerulus and almost completely secreted by the renal tubules. PAH is the ideal marker because it undergoes both filtration and extensive tubular secretion, resulting in nearly 90-100% extraction from the blood in a single pass through the kidneys. Therefore, the clearance of PAH ($C_{PAH}$) is equal to the Effective Renal Plasma Flow (ERPF). **Analysis of Incorrect Options:** * **Inulin (A):** It is the gold standard for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered but neither reabsorbed nor secreted by the tubules. * **Creatinine (C):** An endogenous marker used to estimate GFR in clinical practice. It is slightly secreted by tubules, making it less accurate than Inulin but more practical. * **Mannitol (D):** Like Inulin, Mannitol is used to measure GFR and Extracellular Fluid (ECF) volume, but it is not used for renal perfusion. **High-Yield Clinical Pearls for NEET-PG:** * **Fick’s Principle:** The measurement of RPF is based on this principle. * **Renal Blood Flow (RBF) Calculation:** $RBF = \frac{RPF}{1 - Hematocrit}$. * **Filtration Fraction (FF):** $FF = \frac{GFR}{RPF}$. Normal value is approximately 20% (0.2). * **Extraction Ratio:** PAH has the highest extraction ratio (~0.9) among all substances mentioned.

Question 360: All of the following are true about juxtamedullary nephrons except?

A. It forms 15% of total nephrons.
B. It has a long loop of Henle.
C. It has a slow rate of filtration. (Correct Answer)
D. Major function is concentration of urine by counter-current mechanism.

Explanation: **Explanation:** In the human kidney, there are two types of nephrons: **Cortical nephrons** (85%) and **Juxtamedullary (JM) nephrons** (15%). **Why Option C is the correct (False) statement:** Juxtamedullary nephrons actually have a **higher Glomerular Filtration Rate (GFR)** compared to cortical nephrons. This is due to their larger glomeruli and higher hydrostatic pressure within the glomerular capillaries. Therefore, stating they have a "slow rate of filtration" is physiologically incorrect. **Analysis of other options:** * **Option A:** It is a standard anatomical fact that JM nephrons constitute approximately **15%** of the total nephron population. * **Option B:** JM nephrons are characterized by **long loops of Henle** that extend deep into the renal medulla, reaching the tips of the renal papillae. * **Option D:** Their primary physiological role is the **concentration of urine**. The long loops of Henle, along with the associated **Vasa Recta** (specialized peritubular capillaries), are essential for maintaining the medullary osmotic gradient via the **counter-current mechanism**. **High-Yield Clinical Pearls for NEET-PG:** * **Vasa Recta:** These are only found associated with Juxtamedullary nephrons; cortical nephrons lack them. * **Renin Content:** Juxtamedullary nephrons contain **less renin** than cortical nephrons. * **Function during Stress:** During periods of decreased renal blood flow (like heart failure), blood is shunted from cortical to JM nephrons to maximize water reabsorption.

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

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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

Practice Questions

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