Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 431: Which segment of the nephron has the lowest osmolality under the influence of antidiuretic hormone (ADH)?

A. Descending limb of the loop of Henle
B. Collecting duct
C. Late distal tubule
D. Early distal tubule (Correct Answer)

Explanation: ***Early distal tubule*** - This segment is known as the **cortical diluting segment** because it actively reabsorbs solutes (Na+ and Cl-) via the **Na+-Cl- cotransporter** while being impermeable to water. - This mandatory solute removal ensures the tubular fluid is maximally diluted (hypoosmolar, often around 100 mOsm/L), a process that is independent of **ADH** levels. *Descending limb of the loop of Henle* - This segment is highly permeable to **water** but largely impermeable to solutes, causing water to flow out into the surrounding hypertonic medulla. - Consequently, the osmolality of the tubular fluid **increases** significantly as it moves down toward the loop hairpin turn, making it highly concentrated, not dilute. *Collecting duct* - The influence of **ADH** is to insert **aquaporin 2** channels into the apical membrane, making this segment highly permeable to water. - This allows massive water reabsorption out of the tubule, concentrating the urine and leading to a **high osmolality** within the tubule lumen, especially near the end. *Late distal tubule* - While the fluid here is dilute, some water reabsorption is possible in the presence of **ADH** due to ADH-sensitive aquaporins, similar to the collecting duct. - This water movement slightly increases the osmolality compared to the maximally dilute fluid produced earlier in the **early distal tubule**.

Question 432: What renal function is represented by the formula UV/P?

A. Tubular Secretion
B. Clearance (Correct Answer)
C. Tubular Reabsorption
D. Filtration

Explanation: ***Clearance*** - The formula **(U × V) / P** represents the **renal clearance** (C) of a substance, where U is the urine concentration, V is the urine flow rate, and P is the plasma concentration of that substance - Clearance measures the virtual volume of **plasma** from which a substance is completely removed by the kidney per unit of time (typically expressed in ml/min) - This is the fundamental definition and calculation of renal clearance *Filtration* - Filtration is the bulk flow of fluid and solutes from the glomerular capillaries into Bowman's capsule, quantified by the Glomerular Filtration Rate (GFR) - The net filtration rate of a substance is calculated as GFR multiplied by the plasma concentration (P), which is distinct from the clearance formula UV/P *Tubular Secretion* - Tubular secretion is the active transfer of substances from the peritubular capillaries into the tubular lumen - While UV/P is used to calculate clearance, if a substance's clearance exceeds the GFR, it indicates that net tubular secretion has occurred *Tubular Reabsorption* - Tubular reabsorption is the process where filtered solutes move from the tubular lumen back into the peritubular capillaries - If the clearance (UV/P) of a substance is significantly less than the GFR, it signifies that net reabsorption of the substance has taken place

Question 433: What is the typical Glomerular Filtration Rate (GFR) in a healthy adult?

A. 125 ml/min (Correct Answer)
B. 150 ml/min
C. 100 ml/min
D. 200 ml/min

Explanation: **Correct: 125 ml/min** - This is the accepted average value for **Glomerular Filtration Rate (GFR)** in a standard 70 kg healthy young adult - Represents the total volume of fluid filtered from plasma into Bowman's capsule per minute - Totals approximately **180 liters per day** *Incorrect: 150 ml/min* - Higher than the established average GFR (125 ml/min) - Indicative of **renal hyperfiltration**, often an early sign preceding kidney damage - Not the normal rate for a healthy adult *Incorrect: 100 ml/min* - Within the lower limits of normal for some demographics or mild age-related decline - Not the typical average GFR used in standard physiology - GFR below 90 ml/min suggests potential **renal impairment** (Stage 2 CKD) *Incorrect: 200 ml/min* - Significantly higher than physiological GFR - Suggests **hyperfiltration**, a pathological state associated with conditions like early **diabetes mellitus** - Indicates increased pressure or permeability in glomerular capillaries

Question 434: Which of the following statements is true about GFR? GFR-Glomerular filtration rate RPF-Renal Plasma Flow

A. An increase in sympathetic function increases GFR
B. Afferent arteriolar constriction increases GFR
C. An increase in RPF increases GFR (Correct Answer)
D. A decrease in RPF increases GFR

Explanation: ***An increase in RPF increases GFR***- **GFR** is directly proportional to the amount of **plasma delivered** to the glomerular capillaries for filtration, meaning higher **Renal Plasma Flow (RPF)** generally increases GFR.- Increased RPF contributes to a higher **glomerular hydrostatic pressure** and delivers more solute/fluid load to the capillary surface area for filtration.*An increase in sympathetic function increases GFR*- Strong sympathetic activation causes generalized **vasoconstriction** of the renal blood vessels (via alpha-1 receptors), significantly reducing **RPF** and thus GFR.- This response is critical during conditions like hemorrhage or shock to conserve fluid volume by prioritizing systemic circulation over **renal filtration**.*Afferent arteriolar constriction increases GFR*- Constriction of the **afferent arteriole** restricts blood flow into the glomerulus, immediately dropping the **glomerular capillary hydrostatic pressure (PGC)**.- Since PGC is the chief driving force for filtration, afferent constriction invariably leads to a *decrease* in GFR; this mechanism is essential for **GFR autoregulation**.*A decrease in RPF increases GFR*- A decrease in **Renal Plasma Flow (RPF)** means less plasma is physically available to be filtered across the glomerular membrane per unit time.- A primary reduction in RPF, assuming stable filtration dynamics, generally results in a proportional *decrease* in the absolute **GFR**.

Question 435: Which among the following is true regarding creatinine clearance?

A. Equal to inulin clearance
B. It is less than inulin clearance
C. It is more than inulin clearance (Correct Answer)
D. Equal to renal plasma flow

Explanation: ***It is more than inulin clearance*** - Creatinine clearance **overestimates GFR** by approximately 10-20% compared to inulin clearance - This is because creatinine undergoes both **glomerular filtration AND tubular secretion** - The additional secretion increases the amount of creatinine excreted, making the calculated clearance higher - **Inulin clearance** remains the gold standard as inulin is only filtered (not secreted or reabsorbed) *Equal to inulin clearance* - Incorrect: Creatinine undergoes tubular secretion in addition to filtration, so clearances are not equal *It is less than inulin clearance* - Incorrect: This would only be true if creatinine were reabsorbed, but it is actually secreted, making its clearance higher *Equal to renal plasma flow* - Incorrect: Renal plasma flow is measured by PAH (para-aminohippuric acid) clearance (~650 mL/min), which is much higher than creatinine clearance (~120-130 mL/min) or GFR (~120 mL/min)

Question 436: Which of the following is not a mechanism of action of ADH?

A. Increases absorption of NaCl in thin ascending limb
B. Increases water permeability in collecting ducts
C. Increases absorption of urea in medullary collecting duct
D. Increases absorption of urea in descending limb of loop of Henle (Correct Answer)

Explanation: ***Increases absorption of urea in descending limb of loop of Henle*** - The mechanism of action of **Antidiuretic Hormone (ADH)** does not involve increasing urea absorption in the **descending limb** of the loop of Henle. - The descending limb is primarily permeable to **water only** and lacks ADH-responsive urea transporters. - This is **NOT** a mechanism of ADH action, making this the correct answer. *Increases water permeability in collecting ducts* - ADH binds to **V2 receptors** in the principal cells of the collecting ducts, triggering the insertion of **aquaporin-2 (AQP2)** channels into the luminal membrane. - This is the **primary mechanism** of ADH, allowing water reabsorption and urine concentration. *Increases absorption of urea in medullary collecting duct* - ADH stimulates the insertion of **urea transporters (UT-A1 and UT-A3)** in the inner medullary collecting duct (IMCD). - This passive diffusion of urea into the medullary interstitium helps maintain the high osmolarity required for maximal water reabsorption. - This is an established **direct mechanism** of ADH. *Increases absorption of NaCl in thin ascending limb* - While the thin ascending limb has passive NaCl permeability, ADH's effects on salt handling are primarily mediated through the **thick ascending limb (TAL)** where it enhances Na-K-2Cl cotransporter activity. - ADH contributes to medullary hypertonicity, which indirectly affects the concentration gradient for passive NaCl movement in the thin ascending limb. - This represents an **indirect effect** rather than a primary mechanism, but is still considered an ADH action in generating concentrated urine.

Question 437: Which feedback hormone is marked as $X$ ?

A. Estrogen/Testosterone (Correct Answer)
B. Progesterone
C. Inhibin
D. Activin

Explanation: ***Estrogen/Testosterone*** - The diagram illustrates the **hypothalamic-pituitary-gonadal axis**, where GnRH from the hypothalamus stimulates LH and FSH from the anterior pituitary, which in turn act on the gonads. - The hormone marked 'X' represents the **negative feedback** from the gonadal hormones (estrogen in females, testosterone in males) on the anterior pituitary and hypothalamus, inhibiting further release of LH, FSH, and GnRH. *Progesterone* - While progesterone is a gonadal hormone and exerts negative feedback, it is typically involved alongside estrogen in the **luteal phase** of the menstrual cycle, and 'X' represents the general feedback from the primary sex steroids. - In males, progesterone is not the primary feedback hormone. *Inhibin* - Inhibin is a peptide hormone produced by the gonads that primarily exerts **negative feedback on FSH secretion** from the anterior pituitary, not on LH or GnRH to the same extent. - The feedback denoted by 'X' appears to be a **broader inhibitory signal** encompassing both LH and FSH regulation. *Activin* - Activin is a protein complex that generally **stimulates FSH secretion** from the anterior pituitary, playing an antagonistic role to inhibin. - It does not exert negative feedback on the pituitary or hypothalamus, making it an unlikely candidate for 'X'.

Question 438: A patient has a blood glucose level of $200 \mathrm{mg} \%$ and GFR of 90. The transport maximum of the patient is as shown in the picture given below. What is the amount of glucose excreted?

A. 30 mg/min (Correct Answer)
B. 50 mg/min
C. 90 mg/min
D. 180 mg/min

Explanation: ***30 mg/min*** - The amount of **glucose filtered** by the kidneys is calculated as GFR × plasma glucose concentration, which is 90 mL/min × 2 mg/mL = **180 mg/min**. - The **transport maximum (Tm)** for glucose is 150 mg/min, meaning the kidneys can reabsorb a maximum of 150 mg/min of glucose per minute. - Since more glucose is filtered (180 mg/min) than can be reabsorbed (150 mg/min), the excess is excreted in urine. - **Glucose excreted** = Filtered - Reabsorbed = 180 mg/min - 150 mg/min = **30 mg/min**. *Incorrect: 50 mg/min* - This would be incorrect as it doesn't match the calculation based on the given values. *Incorrect: 90 mg/min* - This represents the GFR value, not the amount of glucose excreted. *Incorrect: 180 mg/min* - This represents the total amount of glucose filtered, not the amount excreted after reabsorption.

Question 439: Which of the following will occur on efferent arteriolar constriction?

A. Increased GFR (Correct Answer)
B. Decreased GFR
C. Increase flow in vasa recta
D. Decreased flow in vasa recta

Explanation: ***Increased GFR*** - **Efferent arteriolar constriction** increases resistance to blood flow out of the glomerulus, causing blood to 'back up' and increase the **glomerular hydrostatic pressure (PGC)** - Higher **glomerular hydrostatic pressure** leads to increased net filtration pressure, which directly results in **increased glomerular filtration rate (GFR)** - This is the **primary and most clinically significant effect** of efferent arteriolar constriction - The increased PGC helps maintain GFR even when renal blood flow decreases slightly *Decreased GFR* - This is incorrect because **efferent arteriolar constriction** increases glomerular hydrostatic pressure, thereby **increasing GFR**, not decreasing it - Only severe efferent constriction that critically reduces renal plasma flow would eventually decrease GFR - The immediate and primary effect is always an increase in GFR *Increase flow in vasa recta* - This is incorrect; **efferent arteriolar constriction** actually **decreases** flow to the vasa recta - The constriction reduces blood flow exiting the glomerulus, which means less blood reaches the downstream peritubular capillaries and vasa recta - This decreased perfusion of the vasa recta can enhance urine concentration by reducing washout of the medullary concentration gradient *Decreased flow in vasa recta* - While this is physiologically true (efferent constriction does reduce peritubular and vasa recta blood flow), it is **not the primary or most significant effect** being tested - In the context of efferent arteriolar constriction, the **increased GFR** is the dominant and most clinically relevant consequence - The question asks "which will occur" expecting the primary hemodynamic effect on glomerular function - Decreased vasa recta flow is a secondary consequence, whereas increased GFR is the direct and immediate result

Question 440: The countercurrent mechanism is present in which of the following parts of the body? A. Eye B. Testes C. Kidney D. Gut E. Lungs

A. A, B, C, D, E are correct
B. B and C are correct (Correct Answer)
C. B, C, D, E are correct
D. A, B, C are correct

Explanation: ***B and C are correct*** - The **countercurrent mechanism** is a biological process where two fluids flow in opposite directions across a semipermeable membrane or in close proximity to maximize exchange efficiency. - **Testes (B)**: The **pampiniform plexus** of veins surrounds the testicular artery, creating a countercurrent heat exchange system. Warm arterial blood descending to the testes is cooled by cooler venous blood ascending from the testes, maintaining the 2-3°C lower temperature required for optimal spermatogenesis. - **Kidney (C)**: The **loop of Henle** and **vasa recta** employ countercurrent multiplication and countercurrent exchange mechanisms to establish and maintain the corticomedullary osmotic gradient, which is essential for concentrating urine. *A, B, C, D, E are correct* - **Eye (A)**: The eye does not have a well-established countercurrent mechanism. Aqueous humor circulation and retinal blood flow do not operate on countercurrent principles. - **Gut (D)**: While intestinal villi have circulation, countercurrent exchange is not a primary or well-established mechanism in standard physiology teaching for the gut. - **Lungs (E)**: The lungs use **concurrent flow**, not countercurrent exchange. Pulmonary capillary blood and alveolar air flow in the same direction, which is less efficient than countercurrent but still allows adequate gas exchange. *A and B are correct* - This option incorrectly includes **Eye (A)**, which does not have a countercurrent mechanism. - It also excludes the **Kidney (C)**, which is one of the most classic and well-taught examples of countercurrent mechanisms in physiology. *A, B, C are correct* - This option incorrectly includes **Eye (A)**, which does not have a countercurrent mechanism. - While it correctly includes testes and kidney, the inclusion of the eye makes this medically inaccurate. *B, C, D, E are correct* - While **Testes (B)** and **Kidney (C)** definitely have countercurrent mechanisms, the inclusion of **Gut (D)** and **Lungs (E)** is incorrect. - The lungs specifically use concurrent, not countercurrent, gas exchange - this is a fundamental concept in respiratory physiology. - Countercurrent exchange in the gut is not a standard teaching point in medical physiology.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

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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