Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 491: Which of the following best describes the role of antidiuretic hormone (ADH) in the renal system?

A. Increases sodium reabsorption
B. Decreases water reabsorption
C. Increases water reabsorption (Correct Answer)
D. Decreases potassium excretion

Explanation: ***Increases water reabsorption*** - **Antidiuretic hormone (ADH)**, also known as **vasopressin**, primarily acts on the **collecting ducts and late distal tubules** of the kidneys to increase their permeability to water by inserting **aquaporin-2 water channels**. - This action leads to **increased water reabsorption**, helping to concentrate urine and conserve body water, which is crucial for maintaining **fluid balance and blood pressure**. - This is the primary and most important role of ADH in the renal system. *Increases sodium reabsorption* - The primary hormone responsible for increasing **sodium reabsorption** in the kidneys is **aldosterone**, which acts on the principal cells of the collecting ducts and distal tubule. - While ADH can indirectly affect sodium concentration by altering water balance, its **direct action is not on sodium transport**. - ADH focuses on water permeability, not sodium handling. *Decreases water reabsorption* - This describes the **opposite effect** of ADH action. - Decreased water reabsorption would result in **dilute urine** and increased water loss from the body. - Conditions like **diabetes insipidus** (central or nephrogenic), characterized by insufficient ADH secretion or renal unresponsiveness to ADH, lead to decreased water reabsorption and polyuria. *Decreases potassium excretion* - ADH does **not directly regulate potassium excretion**; its main role is focused on water permeability and osmolality regulation. - Hormones like **aldosterone** are key regulators of potassium handling, typically promoting K+ secretion into the urine in exchange for sodium reabsorption in the principal cells. - Potassium homeostasis is independent of ADH's primary mechanism of action.

Question 492: Which part of the nephron is primarily responsible for the reabsorption of glucose?

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

Explanation: ***Proximal convoluted tubule*** - The **proximal convoluted tubule (PCT)** is responsible for the reabsorption of nearly **100% of filtered glucose** and amino acids under normal physiological conditions. - This process is primarily mediated by **sodium-glucose cotransporters (SGLTs)** and **glucose transporters (GLUTs)** on the apical and basolateral membranes, respectively. *Distal convoluted tubule* - The **distal convoluted tubule (DCT)** primarily reabsorbs sodium, chloride, and calcium, and is involved in **fine-tuning electrolyte balance** and acid-base regulation. - While it has some reabsorptive capacity, it plays a negligible role in **glucose reabsorption**. *Loop of Henle* - The **Loop of Henle** is crucial for establishing and maintaining the **medullary osmotic gradient**, which is essential for water reabsorption in the collecting ducts. - Its primary functions include the reabsorption of **water** in the descending limb and **solutes** (especially NaCl) in the ascending limb, but not glucose. *Collecting duct* - The **collecting duct** is primarily involved in **water reabsorption** (regulated by ADH) and urea reabsorption, contributing to the concentration of urine. - It also plays a role in acid-base balance by secreting or reabsorbing **hydrogen ions and bicarbonate**, but it does not reabsorb glucose.

Question 493: A patient with chronic anemia is found to have low erythropoietin levels. Which organ's dysfunction is most directly associated with this finding?

A. Liver
B. Pancreas
C. Kidney (Correct Answer)
D. Spleen

Explanation: ***Kidney*** - The **kidneys** are the primary site of **erythropoietin (EPO) production**, a hormone essential for red blood cell formation. - In chronic kidney disease, damaged kidneys produce insufficient EPO, leading to **renal anemia**. *Liver* - The **liver** is involved in erythropoietin production during fetal development and contributes a small amount in adulthood, but it is not the main regulator in adults. - Liver dysfunction is more commonly associated with issues like **coagulopathy** or **metabolic derangements**, not primarily low EPO. *Pancreas* - The **pancreas** primarily produces hormones like **insulin** and **glucagon**, and digestive enzymes. - It has no significant role in erythropoietin production or the direct regulation of erythropoiesis. *Spleen* - The **spleen** is mainly involved in filtering blood, removing old red blood cells, and immune functions. - It does not produce erythropoietin; its dysfunction might lead to issues like **splenomegaly** or **hemolysis**, but not directly low EPO levels.

Question 494: Which of the following best describes the primary function of aldosterone in the renal system?

A. Increases potassium excretion
B. Increases hydrogen ion secretion
C. Promotes water retention indirectly through sodium reabsorption
D. Promotes sodium reabsorption in the distal nephron (Correct Answer)

Explanation: ***Promotes sodium reabsorption in the distal nephron*** - Aldosterone primarily acts on the **principal cells** of the **collecting ducts** and late distal tubules to increase the activity of **epithelial sodium channels (ENaC)** and Na+/K+-ATPase, leading to increased **sodium reabsorption**. - This reabsorption of sodium is critical for maintaining **extracellular fluid volume** and blood pressure. *Increases potassium excretion* - While aldosterone does increase potassium excretion, this is a **secondary effect** of its primary role in sodium reabsorption. - The reabsorbed sodium creates a more **negative luminal potential**, which drives potassium secretion into the tubular lumen via **renal outer medullary potassium (ROMK) channels**. *Increases hydrogen ion secretion* - Aldosterone has a minor role in increasing **hydrogen ion secretion** by stimulating H+-ATPase in the **intercalated cells** of the collecting ducts. - This is a less prominent effect compared to its significant impact on sodium and potassium handling. *Promotes water retention indirectly through sodium reabsorption* - Water retention occurs due to the **osmotic effect** of reabsorbed sodium, which creates a gradient for water to follow. - However, the direct action of aldosterone is on **sodium reabsorption**, and water permeability in the collecting ducts is primarily regulated by **antidiuretic hormone (ADH)**.

Question 495: Diabetes insipidus is diagnosed when:

A. Urine output exceeds 2 liters per day and urine osmolarity is less than 250 mOsm/L
B. Urine output exceeds 3 liters per day and urine osmolarity is less than 300 mOsm/L (Correct Answer)
C. Urine output exceeds 5 liters per day and urine osmolarity is less than 320 mOsm/L
D. Urine output exceeds 4 liters per day and urine osmolarity is less than 280 mOsm/L

Explanation: ***Urine output exceeds 3 liters per day and urine osmolarity is less than 300 mOsm/L*** - **Diabetes insipidus** is characterized by the kidneys' inability to reabsorb water, leading to the excretion of large volumes of dilute urine, typically **more than 3 liters per day**. - This excessive urination is accompanied by a **low urine osmolarity**, generally **less than 300 mOsm/L**, indicating impaired concentrating ability. *Urine output exceeds 2 liters per day and urine osmolarity is less than 250 mOsm/L* - While a urine output **exceeding 2 liters per day** can be considered polyuria, the threshold for diagnosing diabetes insipidus is typically higher, usually **above 3 liters per day**. - A urine osmolarity **less than 250 mOsm/L** is consistent with dilute urine, but the volume criterion is slightly lower than the generally accepted diagnostic cut-off. *Urine output exceeds 5 liters per day and urine osmolarity is less than 320 mOsm/L* - A urine output **exceeding 5 liters per day** certainly indicates severe polyuria, but it's a more extreme presentation rather than the general diagnostic criteria. - While **less than 320 mOsm/L** is dilute, the common threshold for urine osmolarity in diabetes insipidus is often lower, and the volume criterion is also higher than the standard. *Urine output exceeds 4 liters per day and urine osmolarity is less than 280 mOsm/L* - A urine output **exceeding 4 liters per day** is significant but still slightly higher than the commonly cited general threshold of 3 liters per day for typical DI diagnosis. - A urine osmolarity **less than 280 mOsm/L** is indicative of dilute urine and is within the expected range for DI, but the volume criterion is a bit more stringent than general guidelines.

Question 496: Which of the following statements is true regarding the function of the distal convoluted tubule?

A. Reabsorbs chloride ions via sodium-chloride symporters.
B. Reabsorbs sodium primarily via sodium-chloride symporters. (Correct Answer)
C. Reabsorbs water in response to ADH.
D. All of the options.

Explanation: ***Reabsorbs sodium primarily via sodium-chloride symporters*** - This is the **hallmark function** of the distal convoluted tubule (DCT) - The DCT contains the **thiazide-sensitive Na-Cl cotransporter (NCC)**, which is the primary mechanism for **sodium and chloride reabsorption** in this segment - This transporter is clinically significant as it is the target of **thiazide diuretics**, which block NCC and increase sodium and water excretion - The DCT reabsorbs approximately **5-10% of filtered sodium** via this mechanism - This is the **most characteristic and important function** that distinguishes the DCT from other nephron segments *Reabsorbs chloride ions via sodium-chloride symporters* - While this statement is **technically correct**, it is **incomplete** because chloride reabsorption in the DCT is **coupled with sodium** reabsorption - Chloride cannot be reabsorbed independently; it is always co-transported with sodium via the NCC symporter - This option is less comprehensive than stating that sodium is primarily reabsorbed, which is the more clinically and physiologically relevant description *Reabsorbs water in response to ADH* - This statement is **incorrect** for the distal convoluted tubule proper - The **early and mid-DCT are impermeable to water**, even in the presence of ADH (arginine vasopressin) - ADH-mediated water reabsorption primarily occurs in the **collecting duct**, where aquaporin-2 (AQP2) water channels are inserted into the apical membrane - While the **late DCT/connecting segment** may have limited ADH responsiveness, this is **not a primary function** of the DCT and is not characteristic of this nephron segment - Standard physiology teaching classifies the DCT as a **diluting segment** that is water-impermeable *All of the options* - This cannot be correct because the statement about water reabsorption in response to ADH is not accurate for the DCT proper - The DCT's primary and characteristic functions are electrolyte reabsorption, not water reabsorption

Question 497: What is the primary function of Lacis cells in the nephron?

A. Secretion of renin to regulate blood pressure
B. Reabsorption of sodium ions
C. Regulation of blood flow in arterioles
D. Regulation of glomerular filtration rate (Correct Answer)

Explanation: ***Regulation of glomerular filtration rate*** - Lacis cells, also known as extraglomerular mesangial cells, are part of the **juxtaglomerular apparatus** (JGA). - They play a crucial role in the **autoregulation** of the **glomerular filtration rate (GFR)** by transmitting signals between the macula densa and the afferent arteriole. *Secretion of renin to regulate blood pressure* - The primary cells responsible for **renin secretion** are the **juxtaglomerular cells** (granular cells) located in the walls of the afferent arteriole. - While Lacis cells are part of the JGA, their direct role in renin secretion is minimal compared to juxtaglomerular cells. *Reabsorption of sodium ions* - **Sodium reabsorption** primarily occurs in the **renal tubules**, particularly the **proximal tubule** and the **Loop of Henle**. - Lacis cells are not directly involved in the tubular reabsorption of electrolytes. *Regulation of blood flow in arterioles* - While Lacis cells communicate with the afferent and efferent arterioles, their main function is not to directly regulate blood flow but rather to mediate the **tubuloglomerular feedback** mechanism to control GFR. - The **smooth muscle cells** within the arteriolar walls are primarily responsible for regulating blood flow.

Question 498: Plasma inulin of a person is 4 mg/ml and urine flow rate is 20 ml/min. What will be GFR if urine inulin is 50 mg/ml?

A. 125 ml/min
B. 250 ml/min (Correct Answer)
C. 500 ml/min
D. 1000 ml/min

Explanation: ***250 ml/min*** - Glomerular Filtration Rate (GFR) can be calculated using the **clearance formula**: GFR = (Urine Inulin Concentration × Urine Flow Rate) / Plasma Inulin Concentration. - Plugging in the values: (50 mg/ml × 20 ml/min) / 4 mg/ml = 1000 / 4 = **250 ml/min**. *125 ml/min* - This value would be obtained if the urine inulin concentration was lower or if the plasma inulin concentration was higher, resulting in a lower GFR. - For example, if plasma inulin were 8 mg/ml with the other values unchanged, the GFR would be 125 ml/min. *500 ml/min* - This result would be reached if the urine inulin concentration was significantly higher or if the plasma inulin concentration was lower than given. - For instance, if the urine flow rate were 40 ml/min instead of 20 ml/min, GFR would be 500 ml/min. *1000 ml/min* - This GFR is a significantly higher value than typically observed and would require a much larger urine inulin concentration or urine flow rate, or a much lower plasma inulin concentration. - This value would be obtained if the plasma inulin concentration was 1 mg/ml with the given urine inulin and flow rate.

Question 499: Which of the following statements about aquaporins is false?

A. Aquaporins are proteins
B. Aquaporin-1 in PCT
C. Aquaporin-2 in CD
D. Aquaporin-2 in loop of Henle (Correct Answer)

Explanation: ***Aquaporin-2 in loop of Henle (FALSE - Correct Answer)*** - **Aquaporin-2 (AQP2)** is primarily found in the apical membrane of the **collecting duct (CD)**, NOT in the loop of Henle. - Its presence and function in the CD are regulated by **vasopressin (ADH)**, which promotes water reabsorption. - This statement is **false** and is therefore the correct answer to this question. *Aquaporins are proteins (TRUE)* - Aquaporins are indeed a **family of integral membrane proteins** that function as water channels. - They facilitate the rapid movement of water across cell membranes, playing a crucial role in water balance. - This statement is **true**. *Aquaporin-1 in PCT (TRUE)* - **Aquaporin-1 (AQP1)** is abundantly expressed in the **proximal convoluted tubule (PCT)** and the descending limb of the loop of Henle. - It allows for constitutive, unregulated water reabsorption driven by osmotic gradients. - This statement is **true**. *Aquaporin-2 in CD (TRUE)* - **Aquaporin-2 (AQP2)** is correctly located in the apical membrane of the principal cells of the **collecting duct (CD)**. - Its insertion into the membrane and water permeability are regulated by **antidiuretic hormone (ADH)**, making it vital for facultative water reabsorption. - This statement is **true**.

Question 500: What decreases renin secretion?

A. Increased sympathetic stimulation
B. Decreased prostacycline [PGI2]
C. Severe hypotension
D. High NaCl concentration in distal tubules (Correct Answer)

Explanation: ***High NaCl concentration in distal tubules*** - Increased **NaCl concentration** in the **distal tubules** is sensed by the **macula densa** cells, which then **actively inhibit renin release** from the juxtaglomerular cells through adenosine and ATP signaling. - This is part of the **tubuloglomerular feedback mechanism**, a direct negative feedback that reduces glomerular filtration rate and maintains electrolyte balance. - This represents **active suppression** of renin secretion. *Increased sympathetic stimulation* - **Increased sympathetic output** to the kidney, primarily via **beta-1 adrenergic receptors**, directly **stimulates renin secretion**. - This response is part of the body's reaction to stress or hypovolemia to raise blood pressure. *Decreased prostacycline [PGI2]* - **Prostacyclin (PGI2)** is a **vasodilator** that acts locally to **stimulate renin release**. - A **decrease in PGI2** represents **loss of a stimulatory signal** rather than active inhibition of renin. - This is a **permissive factor** - its absence doesn't actively decrease renin, it simply removes one of several stimulatory influences. - The question asks what *decreases* renin, implying active suppression rather than mere absence of stimulation. *Severe hypotension* - **Severe hypotension** leads to a **decrease in renal perfusion pressure**, which is detected by **baroreceptors** in the juxtaglomerular apparatus. - This is a powerful stimulus for **increased renin secretion** via the intrarenal baroreceptor mechanism. - This response is crucial for activating the **renin-angiotensin-aldosterone system (RAAS)** to restore blood pressure.

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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