Renal Physiology Practice Questions

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

Collecting tubules. **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.

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

Urine is hypertonic to plasma. **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.

Q: The normal daily excretion of uric acid ranges from:

500 to 700 mg. ### 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.

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

DTPA. **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.

Q: What structures are included in the juxtaglomerular apparatus?

All of the above. 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.

Question 1

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

Accepted Answer

Collecting tubules

Answer

Proximal convoluted tubules

Answer

Distal convoluted tubules

Answer

Loop of Henle

Question 2

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

Accepted Answer

Urine is hypertonic to plasma

Answer

No secretion of vasopressin

Answer

Urine is hypotonic to plasma

Answer

Urine is isosmotic to plasma

Question 3

All of the following decrease the glomerular filtration rate except?

Accepted Answer

Decreased plasma protein levels

Answer

Increased tubular hydrostatic pressure

Answer

Decreased effective filtration surface area

Answer

Decreased capillary hydrostatic pressure

Question 4

Which of the following statements regarding glomerular capillaries is FALSE?

Accepted Answer

Oncotic pressure is higher in the plasma entering the glomerular capillaries than that within the glomerular capillaries themselves.

Answer

Constriction of the afferent arteriole leads to a fall in glomerular capillary blood pressure.

Answer

The concentration of glucose is equal in glomerular capillaries and the initial glomerular ultrafiltrate.

Answer

Hematocrit is elevated in the glomerular capillaries due to plasma filtration.

Question 5

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

Accepted Answer

Microglobulin

Answer

Lysozyme

Answer

Myoglobin

Answer

Immunoglobulin

Question 6

Free water clearance is positive when the urine is:

Accepted Answer

Dilute

Answer

Concentrated

Answer

Isotonic

Answer

Any of the above

Question 7

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?

Accepted Answer

Creatinine

Answer

K+

Answer

Glucose

Answer

Na+

Question 8

The normal daily excretion of uric acid ranges from:

Accepted Answer

500 to 700 mg

Answer

10 to 30 mg

Answer

50 to 70 mg

Answer

100 to 300 mg

Question 9

Which of the following is selectively filtered into the renal tubules?

Accepted Answer

DTPA

Answer

DMSA

Answer

MAG3

Answer

EC

Question 10

What structures are included in the juxtaglomerular apparatus?

Accepted Answer

All of the above

Answer

Macula densa

Answer

Juxtaglomerular cells

Answer

Extraglomerular mesangial cells

Renal Physiology — MCQs

Renal Physiology — MCQs

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