Renal Physiology — MCQs

Q: What is the primary site of bicarbonate reabsorption in the nephron?

Proximal convoluted tubule. **Explanation** The correct answer is **A. Proximal convoluted tubule (PCT)**. **1. Why the PCT is correct:** The Proximal Convoluted Tubule is the primary site for acid-base regulation in the kidney, responsible for reabsorbing approximately **80–90%** of the filtered bicarbonate ($HCO_3^-$). This process is mediated by the **Na⁺-H⁺ exchanger (NHE3)** on the apical membrane, which secretes $H^+$ into the lumen. The secreted $H^+$ combines with filtered $HCO_3^-$ to form $H_2CO_3$, which is then broken down by **carbonic anhydrase (type IV)** into $CO_2$ and $H_2O$. These molecules diffuse into the cell, are reconstituted into $HCO_3^-$, and transported into the blood via the **Na⁺-$HCO_3^-$ cotransporter (NBCe1)**. **2. Why the other options are incorrect:** * **B. Distal convoluted tubule (DCT):** While some transport occurs here, it is not the primary site. The DCT and collecting ducts handle the remaining 10–15% of bicarbonate. * **C & D. Collecting Ducts:** These segments are primarily responsible for the "fine-tuning" of acid-base balance. **Type A intercalated cells** secrete $H^+$ and reabsorb "new" bicarbonate during acidosis, but the bulk of the filtered load has already been reclaimed by the PCT. **3. High-Yield Clinical Pearls for NEET-PG:** * **Carbonic Anhydrase Inhibitors (Acetazolamide):** These drugs act specifically on the PCT to inhibit bicarbonate reabsorption, leading to alkaline urine and metabolic acidosis. * **Proximal RTA (Type 2):** Caused by a defect in the PCT's ability to reabsorb $HCO_3^-$. * **Threshold:** The renal threshold for $HCO_3^-$ reabsorption is approximately **24–26 mEq/L**; levels above this lead to bicarbonaturia.

Q: What is the clearance of a substance if its concentration in plasma is 10 mg%, concentration in urine is 100 mg%, and urine flow is 2 ml/min?

20 ml/min. ### Explanation **1. Understanding the Correct Answer (D)** Renal clearance is the volume of plasma that is completely cleared of a substance by the kidneys per unit time. It is calculated using the standard clearance formula: **$C = \frac{U \times V}{P}$** * **$U$ (Urine concentration):** 100 mg% (or 100 mg/100 ml) * **$V$ (Urine flow rate):** 2 ml/min * **$P$ (Plasma concentration):** 10 mg% (or 10 mg/100 ml) Plugging in the values: $C = \frac{100 \times 2}{10} = \frac{200}{10} = \mathbf{20\ ml/min}$ The units (mg%) cancel each other out, leaving the final result in ml/min. **2. Why Other Options are Incorrect** * **Option A (0.02 ml/min):** This is a mathematical error likely caused by incorrectly dividing the values or misplacing the decimal point by three places. * **Option B (0.2 ml/min):** This occurs if the formula is inverted ($P / (U \times V)$) or if the urine flow rate is ignored. * **Option C (2 ml/min):** This result would occur if the ratio of $U/P$ was 1, meaning the substance was neither concentrated nor diluted by the kidney. **3. NEET-PG High-Yield Clinical Pearls** * **Inulin Clearance:** The gold standard for measuring **GFR** because it is freely filtered but neither reabsorbed nor secreted. * **Creatinine Clearance:** Used clinically to estimate GFR; it slightly **overestimates** GFR because a small amount is secreted in the tubules. * **PAH (Para-aminohippuric acid) Clearance:** Used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and almost completely secreted. * **Glucose Clearance:** Normally **zero** because it is 100% reabsorbed in the proximal tubule (up to the transport maximum, $T_m$).

Q: Rapid diffusion of water across cell membranes depends on the presence of water channels, called aquaporins. Which aquaporin is present in the proximal convoluted tubule?

Aquaporin 1. **Explanation:** The correct answer is **Aquaporin 1 (AQP1)**. In the kidney, approximately 65% of filtered water is reabsorbed in the **Proximal Convoluted Tubule (PCT)**. This rapid, constitutive movement of water occurs via AQP1 channels located on both the apical and basolateral membranes, as well as the descending limb of the Loop of Henle. Unlike other segments, AQP1 in the PCT is **not regulated by ADH** (Vasopressin), ensuring constant water reabsorption coupled with solute transport. **Analysis of Incorrect Options:** * **Aquaporin 2 (AQP2):** Found exclusively in the **principal cells** of the collecting ducts. It is the only aquaporin regulated by **ADH**. ADH binding to V2 receptors causes the insertion of AQP2 into the apical membrane. * **Aquaporin 5 (AQP5):** Primarily located in **secretory glands** (salivary, lacrimal, and sweat glands) and alveolar type I cells in the lungs. It is not a major renal aquaporin. * **Aquaporin 9 (AQP9):** Functions as an "aquaglyceroporin" (transporting water and glycerol) and is mainly expressed in the **liver**, leukocytes, and brain. **High-Yield NEET-PG Pearls:** * **AQP1:** PCT and Thin Descending Limb (responsible for "obligatory" water reabsorption). * **AQP2:** Apical membrane of Collecting Duct (target of ADH; deficiency causes **Nephrogenic Diabetes Insipidus**). * **AQP3 & AQP4:** Basolateral membrane of Collecting Duct (provide the exit pathway for water into the interstitium). * **Mnemonic:** "AQP**1** is **First** (PCT), AQP**2** is **Two**-wards the end (Collecting Duct)."

Q: Type 1 glomus cells secrete neurotransmitters in response to oxygen levels due to the function of which channel?

K+ channel. **Explanation:** The **Type 1 Glomus cells** (chief cells) of the carotid and aortic bodies act as peripheral chemosensors. The primary mechanism of oxygen sensing involves **Oxygen-sensitive Potassium (K+) channels**. **Mechanism of Action:** 1. **Hypoxia:** When arterial $PO_2$ falls, these oxygen-sensitive K+ channels **close**. 2. **Depolarization:** The reduction in K+ efflux leads to the accumulation of positive charge inside the cell, causing membrane depolarization. 3. **Neurotransmitter Release:** Depolarization opens voltage-gated **Calcium (Ca²⁺) channels**, leading to an influx of Calcium and subsequent exocytosis of neurotransmitters (primarily **Dopamine**, but also ACh and ATP). 4. **Afferent Signaling:** These transmitters stimulate the glossopharyngeal (CN IX) and vagus (CN X) nerves to increase the respiratory rate. **Analysis of Options:** * **D (Correct):** K+ channels are the "sensors" that initiate the electrical response to hypoxia. * **A (Incorrect):** While Na+ channels are involved in action potential propagation, they are not the primary oxygen-sensing mechanism. * **B (Incorrect):** Cl- channels do not play a significant role in the initial depolarization phase of glomus cells. * **C (Incorrect):** Ca²⁺ channels are the *effectors* that lead to neurotransmitter release, but they open only *after* the K+ channels have closed and the cell has depolarized. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Carotid bodies (bifurcation of common carotid) and Aortic bodies (arch of aorta). * **Stimuli:** Peripheral chemoreceptors respond to **decreased $PO_2$** (<60 mmHg), **increased $PCO_2$**, and **decreased pH**. * **Nerve Supply:** Carotid body via **Hering’s nerve** (branch of CN IX); Aortic body via CN X. * **Key Difference:** Central chemoreceptors (medulla) do **not** respond to hypoxia; they respond primarily to changes in $H^+$ concentration in the CSF derived from arterial $CO_2$.

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 1: What is the primary site of bicarbonate reabsorption in the nephron?

A. Proximal convoluted tubule (Correct Answer)
B. Distal convoluted tubule
C. Cortical collecting duct
D. Medullary collecting duct

Explanation: **Explanation** The correct answer is **A. Proximal convoluted tubule (PCT)**. **1. Why the PCT is correct:** The Proximal Convoluted Tubule is the primary site for acid-base regulation in the kidney, responsible for reabsorbing approximately **80–90%** of the filtered bicarbonate ($HCO_3^-$). This process is mediated by the **Na⁺-H⁺ exchanger (NHE3)** on the apical membrane, which secretes $H^+$ into the lumen. The secreted $H^+$ combines with filtered $HCO_3^-$ to form $H_2CO_3$, which is then broken down by **carbonic anhydrase (type IV)** into $CO_2$ and $H_2O$. These molecules diffuse into the cell, are reconstituted into $HCO_3^-$, and transported into the blood via the **Na⁺-$HCO_3^-$ cotransporter (NBCe1)**. **2. Why the other options are incorrect:** * **B. Distal convoluted tubule (DCT):** While some transport occurs here, it is not the primary site. The DCT and collecting ducts handle the remaining 10–15% of bicarbonate. * **C & D. Collecting Ducts:** These segments are primarily responsible for the "fine-tuning" of acid-base balance. **Type A intercalated cells** secrete $H^+$ and reabsorb "new" bicarbonate during acidosis, but the bulk of the filtered load has already been reclaimed by the PCT. **3. High-Yield Clinical Pearls for NEET-PG:** * **Carbonic Anhydrase Inhibitors (Acetazolamide):** These drugs act specifically on the PCT to inhibit bicarbonate reabsorption, leading to alkaline urine and metabolic acidosis. * **Proximal RTA (Type 2):** Caused by a defect in the PCT's ability to reabsorb $HCO_3^-$. * **Threshold:** The renal threshold for $HCO_3^-$ reabsorption is approximately **24–26 mEq/L**; levels above this lead to bicarbonaturia.

Question 2: What is the clearance of a substance if its concentration in plasma is 10 mg%, concentration in urine is 100 mg%, and urine flow is 2 ml/min?

A. 0.02 ml/min
B. 0.2 ml/min
C. 2 ml/min
D. 20 ml/min (Correct Answer)

Explanation: ### Explanation **1. Understanding the Correct Answer (D)** Renal clearance is the volume of plasma that is completely cleared of a substance by the kidneys per unit time. It is calculated using the standard clearance formula: **$C = \frac{U \times V}{P}$** * **$U$ (Urine concentration):** 100 mg% (or 100 mg/100 ml) * **$V$ (Urine flow rate):** 2 ml/min * **$P$ (Plasma concentration):** 10 mg% (or 10 mg/100 ml) Plugging in the values: $C = \frac{100 \times 2}{10} = \frac{200}{10} = \mathbf{20\ ml/min}$ The units (mg%) cancel each other out, leaving the final result in ml/min. **2. Why Other Options are Incorrect** * **Option A (0.02 ml/min):** This is a mathematical error likely caused by incorrectly dividing the values or misplacing the decimal point by three places. * **Option B (0.2 ml/min):** This occurs if the formula is inverted ($P / (U \times V)$) or if the urine flow rate is ignored. * **Option C (2 ml/min):** This result would occur if the ratio of $U/P$ was 1, meaning the substance was neither concentrated nor diluted by the kidney. **3. NEET-PG High-Yield Clinical Pearls** * **Inulin Clearance:** The gold standard for measuring **GFR** because it is freely filtered but neither reabsorbed nor secreted. * **Creatinine Clearance:** Used clinically to estimate GFR; it slightly **overestimates** GFR because a small amount is secreted in the tubules. * **PAH (Para-aminohippuric acid) Clearance:** Used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and almost completely secreted. * **Glucose Clearance:** Normally **zero** because it is 100% reabsorbed in the proximal tubule (up to the transport maximum, $T_m$).

Question 3: Rapid diffusion of water across cell membranes depends on the presence of water channels, called aquaporins. Which aquaporin is present in the proximal convoluted tubule?

A. Aquaporin 1 (Correct Answer)
B. Aquaporin 2
C. Aquaporin 5
D. Aquaporin 9

Explanation: **Explanation:** The correct answer is **Aquaporin 1 (AQP1)**. In the kidney, approximately 65% of filtered water is reabsorbed in the **Proximal Convoluted Tubule (PCT)**. This rapid, constitutive movement of water occurs via AQP1 channels located on both the apical and basolateral membranes, as well as the descending limb of the Loop of Henle. Unlike other segments, AQP1 in the PCT is **not regulated by ADH** (Vasopressin), ensuring constant water reabsorption coupled with solute transport. **Analysis of Incorrect Options:** * **Aquaporin 2 (AQP2):** Found exclusively in the **principal cells** of the collecting ducts. It is the only aquaporin regulated by **ADH**. ADH binding to V2 receptors causes the insertion of AQP2 into the apical membrane. * **Aquaporin 5 (AQP5):** Primarily located in **secretory glands** (salivary, lacrimal, and sweat glands) and alveolar type I cells in the lungs. It is not a major renal aquaporin. * **Aquaporin 9 (AQP9):** Functions as an "aquaglyceroporin" (transporting water and glycerol) and is mainly expressed in the **liver**, leukocytes, and brain. **High-Yield NEET-PG Pearls:** * **AQP1:** PCT and Thin Descending Limb (responsible for "obligatory" water reabsorption). * **AQP2:** Apical membrane of Collecting Duct (target of ADH; deficiency causes **Nephrogenic Diabetes Insipidus**). * **AQP3 & AQP4:** Basolateral membrane of Collecting Duct (provide the exit pathway for water into the interstitium). * **Mnemonic:** "AQP**1** is **First** (PCT), AQP**2** is **Two**-wards the end (Collecting Duct)."

Question 4: Type 1 glomus cells secrete neurotransmitters in response to oxygen levels due to the function of which channel?

A. Na+ channel
B. Cl- channel
C. Ca+2 channel
D. K+ channel (Correct Answer)

Explanation: **Explanation:** The **Type 1 Glomus cells** (chief cells) of the carotid and aortic bodies act as peripheral chemosensors. The primary mechanism of oxygen sensing involves **Oxygen-sensitive Potassium (K+) channels**. **Mechanism of Action:** 1. **Hypoxia:** When arterial $PO_2$ falls, these oxygen-sensitive K+ channels **close**. 2. **Depolarization:** The reduction in K+ efflux leads to the accumulation of positive charge inside the cell, causing membrane depolarization. 3. **Neurotransmitter Release:** Depolarization opens voltage-gated **Calcium (Ca²⁺) channels**, leading to an influx of Calcium and subsequent exocytosis of neurotransmitters (primarily **Dopamine**, but also ACh and ATP). 4. **Afferent Signaling:** These transmitters stimulate the glossopharyngeal (CN IX) and vagus (CN X) nerves to increase the respiratory rate. **Analysis of Options:** * **D (Correct):** K+ channels are the "sensors" that initiate the electrical response to hypoxia. * **A (Incorrect):** While Na+ channels are involved in action potential propagation, they are not the primary oxygen-sensing mechanism. * **B (Incorrect):** Cl- channels do not play a significant role in the initial depolarization phase of glomus cells. * **C (Incorrect):** Ca²⁺ channels are the *effectors* that lead to neurotransmitter release, but they open only *after* the K+ channels have closed and the cell has depolarized. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Carotid bodies (bifurcation of common carotid) and Aortic bodies (arch of aorta). * **Stimuli:** Peripheral chemoreceptors respond to **decreased $PO_2$** (<60 mmHg), **increased $PCO_2$**, and **decreased pH**. * **Nerve Supply:** Carotid body via **Hering’s nerve** (branch of CN IX); Aortic body via CN X. * **Key Difference:** Central chemoreceptors (medulla) do **not** respond to hypoxia; they respond primarily to changes in $H^+$ concentration in the CSF derived from arterial $CO_2$.

Question 5: Which is the most sensitive index for renal tubular function?

A. Specific gravity of urine (Correct Answer)
B. Blood urea
C. Glomerular Filtration Rate (GFR)
D. Creatinine clearance

Explanation: **Explanation:** The correct answer is **A. Specific gravity of urine.** **Why it is the correct answer:** The primary function of the renal tubules (specifically the loop of Henle and the collecting ducts) is to concentrate or dilute urine based on the body's hydration status. This process, known as **urinary concentrating ability**, is one of the earliest functions to be lost in tubular damage (e.g., Chronic Interstitial Nephritis or Acute Tubular Necrosis). Specific gravity measures the density of urine compared to water and reflects the concentration of dissolved solutes. A fixed specific gravity (isosthenuria, ~1.010) indicates that the tubules can no longer concentrate or dilute the glomerular filtrate, making it the most sensitive index for early tubular dysfunction. **Why the other options are incorrect:** * **B. Blood Urea:** This is a marker of renal function but is highly non-specific. It can be elevated due to high protein intake, GI bleed, or dehydration (pre-renal factors) even when tubular function is normal. * **C. Glomerular Filtration Rate (GFR):** GFR is the best overall index of **glomerular function** and total functioning renal mass, but it does not specifically measure the integrity of the tubular epithelium. * **D. Creatinine Clearance:** This is used to estimate GFR. While it is a standard clinical tool for monitoring kidney disease progression, it reflects glomerular filtration rather than tubular reabsorptive or secretory capacity. **NEET-PG High-Yield Pearls:** * **Isosthenuria:** A fixed specific gravity of **1.010** (equal to plasma osmolality) is a hallmark of advanced renal tubular damage. * **Water Deprivation Test:** This is the gold standard clinical test to assess tubular concentrating ability. * **Osmolality vs. Specific Gravity:** While specific gravity is the most *sensitive* bedside index, **Urine Osmolality** is the most *accurate* measure of urine concentration because it depends only on the number of particles, not their size or weight.

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

Practice Questions

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