Renal Physiology Practice Questions

Q: What types of cells are present in the cortical collecting duct?

Both Principal (P) cells and Intercalated (I) cells.. The **Cortical Collecting Duct (CCD)** is a crucial segment of the distal nephron responsible for the fine-tuning of water, electrolyte, and acid-base balance. It is histologically distinct because it contains two specialized cell types: 1. **Principal (P) Cells (approx. 60-70%):** These are the primary sites for **sodium (Na+) reabsorption** and **potassium (K+) secretion**. They are the main targets for **Aldosterone** (which increases Na+/K+ exchange via ENaC channels) and **Antidiuretic Hormone (ADH/Vasopressin)** (which inserts Aquaporin-2 channels for water reabsorption). 2. **Intercalated (I) Cells (approx. 30-40%):** These cells are essential for **acid-base regulation**. * **Type A (Alpha) cells:** Secrete H+ and reabsorb HCO3- (active during acidosis). * **Type B (Beta) cells:** Secrete HCO3- and reabsorb H+ (active during alkalosis). **Why other options are incorrect:** * **Options B & C:** These are incorrect because the CCD is a heterogeneous segment. While P cells are more numerous, I cells are interspersed among them to maintain pH homeostasis. * **Option D:** This is factually incorrect as the CCD is a highly metabolically active epithelial structure. **High-Yield Clinical Pearls for NEET-PG:** * **Potassium-Sparing Diuretics:** Spironolactone and Amiloride act specifically on the **Principal cells** of the CCD. * **Distal Renal Tubular Acidosis (Type 1 RTA):** Caused by a functional defect in the **Alpha-intercalated cells**, leading to an inability to secrete H+ ions. * **Liddle’s Syndrome:** Involves overactivity of ENaC channels in the **Principal cells**, leading to hypertension and hypokalemia.

Q: In the absence of vasopressin, what is the greatest fraction of filtered water absorbed?

Proximal tubule.. **Explanation:** The correct answer is **Proximal tubule (A)**. The absorption of water in the nephron is divided into two types: **obligatory** and **facultative**. 1. **Proximal Convoluted Tubule (PCT):** Approximately **65-70%** of filtered water is reabsorbed here regardless of the body's hydration status or the presence of Antidiuretic Hormone (ADH/Vasopressin). This is "obligatory" water reabsorption, driven by the active transport of sodium (iso-osmotic reabsorption). Even in the complete absence of vasopressin (e.g., Diabetes Insipidus), the PCT remains the site where the largest fraction of water is reclaimed. **Why other options are incorrect:** * **Loop of Henle (B):** Reabsorbs about 15% of filtered water, primarily in the thin descending limb. The ascending limb is impermeable to water. * **Distal Tubule (C) & Cortical Collecting Duct (D):** These segments are responsible for "facultative" water reabsorption. Their permeability to water is strictly dependent on **Vasopressin (ADH)** acting on V2 receptors to insert Aquaporin-2 channels. In the absence of vasopressin, these segments become virtually impermeable to water, leading to the excretion of large volumes of dilute urine. **High-Yield Clinical Pearls for NEET-PG:** * **Obligatory Water Reabsorption:** Occurs in the PCT (65%) and Descending Loop of Henle (15%). Total = ~80%. * **Facultative Water Reabsorption:** Occurs in the Late DT and Collecting Ducts (remaining ~20%) under ADH control. * **Descending Limb of Henle:** Permeable to water but impermeable to solutes (concentrating segment). * **Ascending Limb of Henle:** Impermeable to water but active in solute reabsorption (diluting segment). * **Maximum Urine Osmolality:** 1200–1400 mOsm/L (in the presence of ADH). * **Minimum Urine Osmolality:** 30–50 mOsm/L (in the absence of ADH).

Q: What is the approximate renal plasma flow value?

650 mL/min. **Explanation:** The renal blood flow (RBF) and renal plasma flow (RPF) are critical parameters in understanding kidney function. To arrive at the correct answer, we use the following physiological relationships: 1. **Renal Blood Flow (RBF):** The kidneys receive approximately 20–25% of the total cardiac output. Given an average cardiac output of 5 L/min, the RBF is roughly **1200–1250 mL/min**. 2. **Renal Plasma Flow (RPF):** Blood consists of cells and plasma. RPF is the volume of plasma delivered to the kidneys per unit time. It is calculated using the formula: * $RPF = RBF \times (1 - Hematocrit)$ * Assuming a normal hematocrit of 45% (0.45), the plasma fraction is 55% (0.55). * $1250 \text{ mL/min} \times 0.55 \approx \mathbf{687 \text{ mL/min}}$. * In standard medical texts (like Guyton), the approximate value is rounded to **650 mL/min**. **Analysis of Options:** * **Option A (350 mL/min):** This is too low and does not correlate with standard physiological fractions of cardiac output. * **Option C (950 mL/min):** This value is higher than the typical plasma flow but lower than the total blood flow. * **Option D (1250 mL/min):** This represents the total **Renal Blood Flow (RBF)**, not the plasma flow. **High-Yield Pearls for NEET-PG:** * **Effective Renal Plasma Flow (eRPF):** Measured using **Para-aminohippuric acid (PAH)** clearance because it is both filtered and secreted. It is typically ~585–600 mL/min (slightly less than true RPF). * **Glomerular Filtration Rate (GFR):** Normal value is **125 mL/min**. * **Filtration Fraction (FF):** $GFR / RPF = 125 / 650 \approx \mathbf{19-20\%}$. This indicates that 20% of the plasma entering the kidney is filtered into the Bowman's capsule.

Q: The completeness of a 24-hour urine collection is best assessed by the simultaneous measurement of urinary:

Creatinine. **Explanation:** The gold standard for assessing the completeness of a 24-hour urine collection is the measurement of **Urinary Creatinine**. **Why Creatinine is the Correct Answer:** Creatinine is a metabolic byproduct of muscle metabolism (creatine phosphate breakdown). In a healthy individual with stable renal function, the daily production and subsequent urinary excretion of creatinine are remarkably constant. It depends primarily on an individual's muscle mass rather than diet or fluid intake. For an average adult, creatinine excretion is approximately **15–25 mg/kg/day** in males and **10–20 mg/kg/day** in females. If the total 24-hour creatinine measured is significantly lower than these expected values, it indicates an incomplete collection (under-collection). **Why Other Options are Incorrect:** * **A. Volume:** Urine volume is highly variable and depends on hydration status, ADH levels, and intake of diuretics (e.g., caffeine, alcohol). It cannot be used as a constant marker. * **B. Urea:** Urea excretion is heavily influenced by dietary protein intake and the body's catabolic state, making it an unreliable marker for collection integrity. * **D. pH:** Urinary pH fluctuates throughout the day based on the "post-prandial alkaline tide," systemic acid-base balance, and respiratory status. **High-Yield Clinical Pearls for NEET-PG:** * **Creatinine Index:** A value used to relate 24-hour creatinine excretion to height/weight to assess nutritional status. * **Formula:** Expected 24h Creatinine (Male) = $28 - (0.2 \times \text{age}) \text{ mg/kg/day}$. * **Clinical Use:** 24-hour collections are commonly used to measure **Creatinine Clearance ($C_{cr}$)** to estimate GFR and to quantify **Proteinuria** (e.g., in Nephrotic syndrome or Preeclampsia).

Q: The neonatal kidney achieves concentrating ability equivalent to the adult kidney by what age?

One year of age. **Explanation:** The neonatal kidney is anatomically complete at birth (nephrogenesis ends at 34-36 weeks of gestation), but it is functionally immature. The ability to concentrate urine depends on the **medullary osmotic gradient** and the responsiveness of the distal tubules to **Antidiuretic Hormone (ADH)**. **1. Why Option A is Correct:** While glomerular filtration rate (GFR) increases rapidly after birth, the concentrating capacity matures more slowly. At birth, a neonate can only concentrate urine to approximately 500–700 mOsm/L. This limitation is due to shorter Loops of Henle, lower urea concentration in the medulla, and relative insensitivity to ADH. By **one year of age**, the tubular function and medullary gradient mature sufficiently to reach the adult concentrating capacity of approximately **1200–1400 mOsm/L**. **2. Why Other Options are Incorrect:** * **Option B:** By 18 months, renal function is well-established, but the specific physiological milestone for adult-level concentration is typically reached by the first birthday. * **Option C:** At 3 to 6 months, GFR and some tubular functions are improving, but the kidney still cannot handle high solute loads as effectively as an adult kidney. * **Option D:** Puberty is far too late; most renal parameters (GFR, secretion, and concentration) reach adult levels between 1 and 2 years of age. **High-Yield Clinical Pearls for NEET-PG:** * **GFR Milestone:** GFR reaches adult levels (adjusted for body surface area) by **2 years of age**. * **Neonatal Vulnerability:** Because neonates cannot concentrate urine effectively, they are at a much higher risk of **dehydration** and hypernatremia during periods of fluid loss (e.g., diarrhea). * **Urea’s Role:** The lower concentrating ability in infants is partly due to their high anabolic state; they use dietary protein for growth, leaving less urea available to contribute to the medullary osmotic gradient.

Q: Renal filtration fraction is calculated as?

Glomerular Filtration Rate (GFR)/Renal plasma flow (RPF). **Explanation:** **1. Understanding the Correct Answer (Option B):** The **Filtration Fraction (FF)** represents the fraction of renal plasma that is actually filtered across the glomerular capillaries into the Bowman’s space. It is mathematically expressed as the ratio of the **Glomerular Filtration Rate (GFR)** to the **Renal Plasma Flow (RPF)**. * **Formula:** $FF = GFR / RPF$ * **Normal Value:** In a healthy adult, GFR is approximately 125 mL/min and RPF is approximately 625 mL/min. Therefore, the normal FF is about **0.20 or 20%**. This means 20% of the plasma entering the kidneys is filtered, while the remaining 80% leaves via the efferent arterioles to become peritubular capillary flow. **2. Why Other Options are Incorrect:** * **Option A:** This is the inverse of the correct formula. RPF/GFR does not represent a standard physiological index. * **Option C:** This formula is mathematically incorrect and does not correlate with any established renal hemodynamic parameter. Total blood flow (RBF) includes hematocrit, whereas filtration only occurs from the plasma component. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Effect of Constriction:** * **Afferent Arteriole Constriction:** Decreases both GFR and RPF; FF remains relatively constant or decreases. * **Efferent Arteriole Constriction:** Decreases RPF but increases GFR (due to increased hydrostatic pressure). Therefore, **FF increases.** * **Clinical Significance:** In states of heart failure or dehydration, RPF drops more significantly than GFR (due to compensatory efferent constriction by Angiotensin II). This leads to an **increased Filtration Fraction**, which helps maintain waste excretion despite low flow. * **Key Relation:** $RPF = RBF \times (1 - \text{Hematocrit})$. Always ensure you use Plasma Flow, not Blood Flow, when calculating FF.

Question 1

Testotoxicosis is due to which of the following?

Accepted Answer

Both gain and loss of function mutations of Gs alpha subunit

Answer

Gain of function mutation of Gs alpha subunit

Answer

Loss of function mutation of Gs alpha subunit

Answer

Mutation in Gs alpha subunit resulting in decreased intrinsic GTPase activity

Question 2

The countercurrent multiplier system is primarily driven by?

Accepted Answer

Concentration of Na+

Answer

H+ at the loop of Henle

Answer

Concentration of Na+ Cl-

Answer

H+ at the ascending loop of Henle and osmolality at the loop of Henle

Question 3

What types of cells are present in the cortical collecting duct?

Accepted Answer

Both Principal (P) cells and Intercalated (I) cells.

Answer

Only P cells.

Answer

Only I cells.

Answer

No cellular content.

Question 4

In the absence of vasopressin, what is the greatest fraction of filtered water absorbed?

Accepted Answer

Proximal tubule.

Answer

Loop of Henle.

Answer

Distal tubule.

Answer

Cortical collecting duct.

Question 5

What is the approximate renal plasma flow value?

Accepted Answer

650 mL/min

Answer

350 mL/min

Answer

950 mL/min

Answer

1250 mL/min

Question 6

Glomerular filtration rate (GFR) is determined by?

Accepted Answer

All of the above

Answer

Afferent arteriolar resistance

Answer

Efferent arteriolar resistance

Answer

Mean arterial pressure

Question 7

The completeness of a 24-hour urine collection is best assessed by the simultaneous measurement of urinary:

Accepted Answer

Creatinine

Answer

Volume

Answer

Urea

Answer

pH

Question 8

Impaired function of aquaporins results in which of the following conditions?

Accepted Answer

Nephrogenic diabetes insipidus

Answer

Liddle syndrome

Answer

Cystic fibrosis

Answer

Baer syndrome

Question 9

The neonatal kidney achieves concentrating ability equivalent to the adult kidney by what age?

Accepted Answer

One year of age

Answer

Eighteen months of age

Answer

Three to six months of age

Answer

Just before puberty

Question 10

Renal filtration fraction is calculated as?

Accepted Answer

Glomerular Filtration Rate (GFR)/Renal plasma flow (RPF)

Answer

Renal plasma flow (RPF)/Glomerular Filtration Rate (GFR)

Answer

(RPF x GFR)/ Total Blood flow

Answer

None of the above

Renal Physiology — MCQs

Renal Physiology — MCQs

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