Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 251: In the nephrons, at which level does vasopressin mainly act?

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

Explanation: **Explanation:** **Correct Answer: D. Collecting tubules** Vasopressin, also known as Antidiuretic Hormone (ADH), is the primary regulator of water reabsorption in the kidney. Its main site of action is the **principal cells of the collecting tubules** (specifically the cortical and medullary collecting ducts). **Mechanism:** ADH binds to **V2 receptors** on the basolateral membrane, triggering a cAMP-mediated signaling pathway. This leads to the insertion of **Aquaporin-2 (AQP2)** water channels into the apical (luminal) membrane. This increases the water permeability of the otherwise impermeable collecting duct, allowing water to be reabsorbed down the osmotic gradient into the hypertonic renal interstitium. **Why other options are incorrect:** * **A. Proximal Convoluted Tubules (PCT):** About 65% of water is reabsorbed here via Aquaporin-1. This process is "obligatory" and occurs isosmotically, independent of ADH. * **B. Distal Convoluted Tubules (DCT):** While the late DCT has some sensitivity to ADH, the **collecting duct** is the primary and most significant site for ADH-mediated water fine-tuning. * **C. Loop of Henle:** The descending limb is permeable to water, but this is mediated by Aquaporin-1 and is not regulated by ADH. The ascending limb is always impermeable to water. **High-Yield Clinical Pearls for NEET-PG:** * **V1 Receptors:** Located on vascular smooth muscle; cause vasoconstriction (Gq protein-coupled). * **V2 Receptors:** Located in the kidney; cause water reabsorption (Gs protein-coupled). * **Diabetes Insipidus (DI):** Central DI is a deficiency of ADH secretion, while Nephrogenic DI is due to resistance at the V2 receptor level. * **SIADH:** Characterized by excessive ADH, leading to water retention and dilutional hyponatremia.

Question 252: Free water clearance of -1.3 ml/min signifies what about the urine?

A. No secretion of vasopressin
B. Urine is hypotonic to plasma
C. Urine is hypertonic to plasma (Correct Answer)
D. Urine is isosmotic to plasma

Explanation: **Explanation:** Free water clearance ($C_{H2O}$) is a measure of the kidney's ability to concentrate or dilute urine. It represents the volume of blood plasma that is cleared of solute-free water per unit time. **1. Why the correct answer is right:** The formula for free water clearance is: $C_{H2O} = V - C_{osm}$ (where $V$ is urine flow rate and $C_{osm}$ is osmolar clearance). * **Negative Free Water Clearance (-1.3 ml/min):** This occurs when $V < C_{osm}$. It signifies that the kidneys are conserving water. To achieve this, the kidneys must reabsorb "free water" back into the systemic circulation, resulting in **concentrated (hypertonic) urine**. This typically occurs under the influence of ADH (Vasopressin). **2. Why the incorrect options are wrong:** * **Option A:** If there were no secretion of vasopressin (as in Diabetes Insipidus), the kidneys would be unable to reabsorb water in the collecting ducts, leading to a **positive** free water clearance. * **Option B:** Hypotonic urine (dilute urine) results in a **positive** free water clearance ($C_{H2O} > 0$), as excess water is being excreted. * **Option D:** Isosmotic urine occurs when $V = C_{osm}$, resulting in a free water clearance of **zero**. **High-Yield Clinical Pearls for NEET-PG:** * **Positive $C_{H2O}$:** Seen in water diuresis, Diabetes Insipidus, and excessive fluid intake. * **Negative $C_{H2O}$:** Seen in dehydration, SIADH, and hemorrhage (states where ADH is high). * **Site of Action:** The generation of free water occurs in the **diluting segments** (Thick Ascending Limb and Early Distal Tubule), but the final determination of $C_{H2O}$ occurs in the **Collecting Ducts** via ADH.

Question 253: All of the following decrease the glomerular filtration rate except?

A. Decreased plasma protein levels (Correct Answer)
B. Increased tubular hydrostatic pressure
C. Decreased effective filtration surface area
D. Decreased capillary hydrostatic pressure

Explanation: To understand this question, we must refer to the **Starling forces** that govern Glomerular Filtration Rate (GFR). The net filtration pressure is determined by the balance between hydrostatic and oncotic pressures: **GFR = Kf × [(Pgc – Pbs) – (πgc – πbs)]** ### 1. Why "Decreased plasma protein levels" is the correct answer: Plasma proteins (primarily albumin) are responsible for **Plasma Colloid Osmotic Pressure (πgc)**. This pressure acts as a "pulling" force that opposes filtration by keeping fluid within the glomerular capillaries. * **Mechanism:** When plasma protein levels decrease (as seen in nephrotic syndrome or liver failure), the opposing oncotic pressure drops. * **Result:** Less opposition to filtration leads to an **increase in GFR**. Therefore, it is the only option that does not decrease GFR. ### 2. Analysis of Incorrect Options (Factors that Decrease GFR): * **Increased tubular hydrostatic pressure (Pbs):** An increase in pressure within Bowman’s space (e.g., due to kidney stones or ureteral obstruction) pushes fluid back toward the capillary, thereby **decreasing GFR**. * **Decreased effective filtration surface area (Kf):** Conditions like mesangial cell contraction or chronic kidney disease reduce the available surface area for exchange, directly **decreasing GFR**. * **Decreased capillary hydrostatic pressure (Pgc):** This is the primary driving force for filtration. A drop in systemic blood pressure or afferent arteriolar constriction reduces this pressure, leading to a **decrease in GFR**. ### 3. High-Yield Clinical Pearls for NEET-PG: * **Afferent Arteriole:** Constriction (by NSAIDs/Sympathetics) decreases GFR; Dilation (by Prostaglandins/ANP) increases GFR. * **Efferent Arteriole:** Constriction (by Angiotensin II) increases GFR (up to a point); Dilation (by ACE inhibitors) decreases GFR. * **Kf (Filtration Coefficient):** This is the product of permeability and surface area. Mesangial cells regulate Kf; their contraction (via ADH or Angiotensin II) reduces surface area and GFR.

Question 254: Which of the following statements regarding glomerular capillaries is FALSE?

A. Oncotic pressure is higher in the plasma entering the glomerular capillaries than that within the glomerular capillaries themselves. (Correct Answer)
B. Constriction of the afferent arteriole leads to a fall in glomerular capillary blood pressure.
C. The concentration of glucose is equal in glomerular capillaries and the initial glomerular ultrafiltrate.
D. Hematocrit is elevated in the glomerular capillaries due to plasma filtration.

Explanation: **Explanation** **1. Why Option A is the Correct (False) Statement:** In the renal corpuscle, plasma undergoes ultrafiltration as it moves from the afferent arteriole through the glomerular capillaries. Since water and small solutes are filtered out while plasma proteins (like albumin) are retained, the **concentration of proteins increases** along the length of the capillary. Consequently, the **colloid osmotic (oncotic) pressure rises** as blood moves from the afferent to the efferent end. Therefore, oncotic pressure is lower at the entrance and higher within/at the exit of the glomerular capillaries. **2. Analysis of Other Options:** * **Option B:** Constriction of the **afferent arteriole** increases resistance before the glomerulus, reducing blood flow and hydrostatic pressure ($P_{GC}$) within the capillaries, leading to a fall in GFR. * **Option C:** Glucose is a small, freely filtered molecule. Its concentration in the initial Bowman’s space (ultrafiltrate) is identical to that in the plasma of the glomerular capillaries (Filtration Fraction for glucose ≈ 1.0). * **Option D:** As plasma is filtered into the Bowman’s space, the ratio of RBCs to the remaining plasma volume increases. This results in an **increased hematocrit** in the blood exiting via the efferent arteriole compared to the afferent arteriole. **High-Yield Clinical Pearls for NEET-PG:** * **Starling Forces:** GFR is governed by $GFR = K_f [(P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS})]$. * **Filtration Equilibrium:** In some species, $\pi_{GC}$ rises until it equals the net hydrostatic pressure, at which point filtration stops (filtration equilibrium). * **Effector of GFR:** Constriction of the **efferent** arteriole *increases* $P_{GC}$ and GFR (up to a point), while constriction of the **afferent** arteriole *decreases* both.

Question 255: Which of the following is normally prevented from passing transglomerularly by the kidney?

A. Microglobulin (Correct Answer)
B. Lysozyme
C. Myoglobin
D. Immunoglobulin

Explanation: The glomerular filtration barrier acts as a selective sieve based on two primary factors: **molecular size** and **electrical charge**. ### Explanation of the Correct Answer **A. Microglobulin (specifically $\beta_2$-microglobulin):** This is the correct answer in the context of "normal prevention" because, while it is small enough to be filtered (~11.8 kDa), it is **completely reabsorbed** by the proximal convoluted tubules under normal physiological conditions. However, in the context of the glomerular basement membrane (GBM) barrier, the question likely refers to larger proteins. If we look at the options strictly by size/charge exclusion, **Immunoglobulins (IgG)** are the only molecules listed that are physically too large to pass through the glomerular pores. *Note: There is a common discrepancy in medical entrance exams regarding this specific question. While $\beta_2$-microglobulin is filtered and reabsorbed, **Immunoglobulins** are the molecules "prevented from passing" the barrier itself due to their high molecular weight (~150 kDa).* ### Why Other Options are Incorrect * **B. Lysozyme:** A small protein (~14 kDa) that is freely filtered at the glomerulus and subsequently reabsorbed by the tubules. * **C. Myoglobin:** A small monomeric protein (~17 kDa). It passes easily through the glomerular filtrate. In clinical conditions like rhabdomyolysis, excess myoglobinuria occurs because the tubular reabsorptive capacity is overwhelmed. * **D. Immunoglobulin:** These are large macromolecules. IgG has a molecular weight of 150,000 Da, which far exceeds the glomerular capillary pore size (cutoff is roughly 70,000 Da, the size of albumin). ### NEET-PG High-Yield Pearls * **Glomerular Barrier Layers:** Fenestrated endothelium, Glomerular Basement Membrane (GBM), and Podocyte slit diaphragms (containing **Nephrin**). * **Charge Selectivity:** The GBM is coated with **Heparan sulfate** (negative charge), which repels negatively charged proteins like Albumin. * **Size Cutoff:** Molecules <4 nm are freely filtered; those >8 nm (like Immunoglobulins) are excluded. * **Minimal Change Disease:** Loss of the negative charge (heparan sulfate) leads to selective proteinuria (albuminuria).

Question 256: Free water clearance is positive when the urine is:

A. Dilute (Correct Answer)
B. Concentrated
C. Isotonic
D. Any of the above

Explanation: **Explanation:** Free water clearance ($C_{H2O}$) is a measure of the kidney's ability to excrete or conserve water independent of solutes. It is calculated using the formula: **$C_{H2O} = V - C_{osm}$** (where $V$ is urine flow rate and $C_{osm}$ is osmolar clearance). **1. Why "Dilute" is correct:** Free water clearance is **positive ($C_{H2O} > 0$)** when the kidneys excrete more water than is required to make the urine isotonic to plasma. This occurs when the urine is **hypotonic (dilute)**, meaning the urine osmolality ($U_{osm}$) is less than the plasma osmolality ($P_{osm}$). This typically happens in the absence of ADH (e.g., high water intake or Diabetes Insipidus), where the distal segments of the nephron remain impermeable to water while continuing to reabsorb solutes. **2. Why other options are incorrect:** * **Concentrated:** When urine is hypertonic ($U_{osm} > P_{osm}$), the kidneys are conserving water. This results in a **negative** free water clearance (also called *free water reabsorption* or $T^c_{H2O}$), indicating that "free water" is being returned to the systemic circulation. * **Isotonic:** If urine is isosmotic to plasma ($U_{osm} = P_{osm}$), the volume of urine excreted is exactly equal to the osmolar clearance. Therefore, $C_{H2O}$ is **zero**. * **Any of the above:** Incorrect, as the sign of $C_{H2O}$ is strictly dependent on the urine-to-plasma osmolality ratio. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Dilution:** The "diluting segments" of the nephron are the **Thick Ascending Limb (TAL)** of the Loop of Henle and the **Early Distal Tubule**. * **Loop Diuretics:** These drugs (like Furosemide) inhibit the Na-K-2Cl symporter in the TAL, interfering with the medullary gradient. Consequently, they decrease both the ability to concentrate urine and the ability to dilute it, bringing $C_{H2O}$ closer to **zero**. * **ADH Influence:** Positive $C_{H2O}$ occurs when ADH levels are low; negative $C_{H2O}$ occurs when ADH levels are high.

Question 257: The GFR of a 26-year-old man with glomerulonephritis decreases by 50% and remains at that level. For which substance would you expect to find the greatest increase in plasma concentration?

A. Creatinine (Correct Answer)
B. K+
C. Glucose
D. Na+

Explanation: ### Explanation **1. Why Creatinine is Correct:** The relationship between GFR and plasma concentration of substances depends on how the kidney handles them. Creatinine is a metabolic byproduct that is **filtered freely but not reabsorbed** (and only minimally secreted). * **The Inverse Relationship:** Plasma creatinine concentration is inversely proportional to GFR. If GFR decreases by 50%, the excretion rate initially drops, causing creatinine to accumulate in the blood until the plasma level roughly **doubles**. At this new higher steady state, the filtered load ($GFR \times P_{Cr}$) matches the production rate again. Because the kidneys have no significant compensatory mechanism to increase creatinine excretion other than filtration, it shows the most significant and predictable rise in plasma concentration. **2. Why the Other Options are Incorrect:** * **B. K+ (Potassium):** While K+ is excreted by the kidneys, the distal nephron (under the influence of **Aldosterone**) can significantly increase secretion per remaining functional nephron. Therefore, plasma K+ levels often remain near normal until GFR drops below 20-25% of normal. * **C. Glucose:** Glucose is not regulated by GFR; its plasma concentration depends on dietary intake and insulin/glucagon balance. It is normally 100% reabsorbed in the proximal tubule unless the renal threshold (180 mg/dL) is exceeded. * **D. Na+ (Sodium):** Sodium balance is tightly regulated by multiple feedback mechanisms (RAAS, ANP, and pressure natriuresis). As GFR drops, fractional excretion of sodium increases, maintaining near-normal plasma levels until end-stage renal failure. **3. High-Yield Clinical Pearls for NEET-PG:** * **Creatinine Blind Zone:** Plasma creatinine may not rise significantly above the normal range until GFR has decreased by nearly 50%. This makes it a specific but **insensitive** marker for early renal disease. * **Steady State Concept:** In chronic renal failure, for substances like Creatinine and Urea, the plasma concentration must rise to maintain an excretion rate equal to the production rate. * **Formula:** $GFR \approx 1 / \text{Plasma Creatinine}$. If GFR is halved, Plasma Cr doubles. If GFR falls to 1/4th, Plasma Cr quadruples.

Question 258: The normal daily excretion of uric acid ranges from:

A. 10 to 30 mg
B. 50 to 70 mg
C. 100 to 300 mg
D. 500 to 700 mg (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. 500 to 700 mg** **Understanding the Concept:** Uric acid is the end product of **purine metabolism** (adenine and guanine) in humans. It is primarily excreted by the kidneys (approx. 70%) and the gastrointestinal tract (approx. 30%). In a healthy adult on a standard diet, the kidneys filter and process uric acid through a complex mechanism of glomerular filtration, proximal tubular reabsorption, and tubular secretion. The net result is a daily urinary excretion of approximately **500 to 700 mg**. This value can fluctuate based on dietary purine intake (e.g., red meat, seafood) and endogenous cell turnover. **Analysis of Incorrect Options:** * **Options A & B (10–70 mg):** These values are significantly lower than the physiological norm. Such low levels would only be seen in states of severe hypouricemia or hereditary renal urate transporter defects, which are rare. * **Option C (100–300 mg):** While this represents a significant amount, it is still below the average daily production and excretion rate for a healthy adult. This range might be seen in individuals on a strictly purine-free diet or those with specific metabolic deficiencies. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Handling:** 100% of uric acid is filtered; 98-100% is reabsorbed in the early proximal tubule; 50% is then secreted in the mid-proximal tubule; and finally, 40-44% is reabsorbed in the distal proximal tubule. The **net excretion is about 8-12%** of the filtered load. * **Hyperuricemia:** Defined as serum levels >7 mg/dL in men and >6 mg/dL in women. It is the primary risk factor for **Gout**. * **Uricosuric Drugs:** Drugs like **Probenecid** increase uric acid excretion by inhibiting the URAT1 transporter in the proximal tubule. * **Enzyme Fact:** Humans lack the enzyme **Uricase**, which converts uric acid to the more soluble allantoin; this is why humans are prone to hyperuricemia compared to other mammals.

Question 259: Which of the following is selectively filtered into the renal tubules?

A. DTPA (Correct Answer)
B. DMSA
C. MAG3
D. EC

Explanation: **Explanation:** The correct answer is **DTPA (Diethylenetriaminepentaacetic acid)**. In renal physiology and nuclear medicine, the handling of radiopharmaceuticals by the nephron depends on their pharmacokinetics. 1. **DTPA (Technetium-99m DTPA):** This agent is handled almost exclusively by **glomerular filtration**. It is not secreted or reabsorbed by the renal tubules. Because its clearance rate is nearly identical to the Glomerular Filtration Rate (GFR), it is the gold standard radiopharmaceutical for measuring GFR and evaluating renal perfusion. **Analysis of Incorrect Options:** * **B. DMSA (Dimercaptosuccinic acid):** This agent binds to the **proximal convoluted tubules** and remains fixed in the renal cortex for a long duration. It is used for **static renal imaging** to detect scars, ectopic kidneys, or pyelonephritis, rather than measuring filtration. * **C. MAG3 (Mercaptoacetyltriglycine):** This is primarily handled by **tubular secretion** (approx. 80%) and only minimally by filtration. It is the agent of choice for dynamic renography in patients with suspected obstruction or renal failure. * **D. EC (Ethylene Dicysteine):** Similar to MAG3, EC is primarily cleared by **tubular secretion**. It offers a better target-to-background ratio but is not a marker for pure filtration. **High-Yield Clinical Pearls for NEET-PG:** * **GFR Marker:** Tc-99m DTPA (Exogenous) and Creatinine/Inulin (Physiological). * **Renal Plasma Flow (RPF) Marker:** PAH (Para-aminohippurate) or Tc-99m MAG3. * **Renal Cortical Imaging:** Tc-99m DMSA. * **Best for Neonates:** MAG3 is preferred over DTPA due to the immature GFR in newborns.

Question 260: What structures are included in the juxtaglomerular apparatus?

A. Macula densa
B. Juxtaglomerular cells
C. Extraglomerular mesangial cells
D. All of the above (Correct Answer)

Explanation: The **Juxtaglomerular Apparatus (JGA)** is a specialized structure located at the point where the distal convoluted tubule (DCT) comes into contact with the afferent arteriole of the same nephron. It plays a critical role in regulating glomerular filtration rate (GFR) and systemic blood pressure. ### Components of the JGA: 1. **Macula Densa:** These are specialized epithelial cells in the initial part of the DCT. They act as **chemoreceptors** that sense changes in sodium chloride (NaCl) concentration in the tubular fluid. 2. **Juxtaglomerular (JG) Cells:** These are modified smooth muscle cells located primarily in the wall of the **afferent arteriole**. They act as **baroreceptors** (sensing pressure changes) and synthesize, store, and release **Renin**. 3. **Extraglomerular Mesangial Cells (Lacis Cells/Polkissen Cells):** Located in the triangular space between the afferent and efferent arterioles and the macula densa. They facilitate signaling between the macula densa and JG cells. ### Why "All of the above" is correct: The JGA is not a single cell type but a functional unit. Since Macula densa (A), JG cells (B), and Extraglomerular mesangial cells (C) are the three anatomical pillars that constitute this apparatus, all options are correct. ### High-Yield NEET-PG Pearls: * **Tubuloglomerular Feedback (TGF):** This is the primary function of the JGA. When NaCl levels rise (indicating high GFR), the macula densa triggers afferent arteriolar constriction to normalize GFR. * **Renin Release:** Stimulated by decreased renal perfusion pressure (sensed by JG cells), increased sympathetic activity ($\beta_1$ receptors), or decreased NaCl delivery to the macula densa. * **Histology Note:** JG cells contain prorenin and renin granules, which stain positive with PAS (Periodic Acid-Schiff) stain.

Practice by Chapter

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Sodium and Water Balance

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Integrative Responses to Fluid Challenges

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