Renal Physiology Practice Questions

Q: Renal plasma flow is best determined by:

PAH. **Explanation:** The measurement of Renal Plasma Flow (RPF) relies on the **Fick Principle**, which states that the amount of a substance entering an organ must equal the amount leaving it. To measure RPF accurately, a substance must be filtered and secreted so efficiently that it is almost entirely cleared from the blood in a single pass through the kidney. **Para-aminohippuric acid (PAH)** is the gold standard for measuring **Effective Renal Plasma Flow (ERPF)** because it is both freely filtered at the glomerulus and actively secreted by the proximal tubules. At low plasma concentrations, approximately 90% of PAH is removed from the renal arterial blood, making its clearance rate a near-accurate reflection of plasma flow. **Analysis of Incorrect Options:** * **Inulin (A):** Inulin is freely filtered but neither reabsorbed nor secreted. Therefore, its clearance is used to measure the **Glomerular Filtration Rate (GFR)**, not RPF. * **Creatinine (B):** An endogenous byproduct of muscle metabolism, it is used for clinical estimation of **GFR**. While it is slightly secreted, it does not provide an accurate measure of total plasma flow. * **Mannitol (D):** Similar to inulin, mannitol is a polysaccharide that is filtered but not significantly secreted or reabsorbed; it is primarily used to measure GFR or ECF volume. **High-Yield Clinical Pearls for NEET-PG:** * **Extraction Ratio:** The extraction ratio of PAH is ~0.9. To find **True Renal Plasma Flow**, use the formula: $ERPF / 0.9$. * **Renal Blood Flow (RBF):** Can be calculated using RPF and Hematocrit (Hct): $RBF = RPF / (1 - Hct)$. * **Filtration Fraction (FF):** The ratio of GFR to RPF ($FF = GFR / RPF$). Normal value is approximately 20%. * **Transport Maximum ($T_m$):** If plasma PAH levels exceed the $T_m$ of the secretory carriers, PAH clearance will decrease and no longer accurately reflect RPF.

Q: Which hormone is secreted by the kidney?

Erythropoietin. **Explanation:** The kidney is not only an excretory organ but also a vital endocrine organ. The correct answer is **Erythropoietin (EPO)**. **1. Why Erythropoietin is correct:** Erythropoietin is a glycoprotein hormone primarily secreted by the **interstitial cells (peritubular capillaries)** in the renal cortex. Its secretion is stimulated by renal hypoxia (low oxygen levels). EPO travels to the bone marrow to stimulate the production of red blood cells (erythropoiesis). **2. Why the other options are incorrect:** * **Angiotensin I:** This is produced in the **bloodstream**. It is formed when Renin (secreted by the kidney) acts on Angiotensinogen (produced by the liver). * **Angiotensin II:** This is primarily formed in the **lungs** (and vascular endothelium) by the action of Angiotensin-Converting Enzyme (ACE) on Angiotensin I. * **Thrombomodulin:** This is an integral membrane protein expressed on the surface of **vascular endothelial cells**; it is not a hormone secreted by the kidney. **3. High-Yield Clinical Pearls for NEET-PG:** * **Chronic Kidney Disease (CKD):** Patients with CKD develop normocytic normochromic anemia primarily due to a deficiency in Erythropoietin. * **Other Renal Hormones:** Besides EPO, the kidney secretes **Renin** (from Juxtaglomerular cells) and **1,25-dihydroxyvitamin D3 (Calcitriol)**, which is the active form of Vitamin D converted by the enzyme 1-alpha-hydroxylase in the proximal tubules. * **Prostaglandins:** The kidney also produces PGE2 and PGI2, which act as local vasodilators to maintain renal blood flow.

Q: The countercurrent mechanism in the kidney is primarily responsible for which of the following?

Creation of an osmotic gradient in the medulla. **Explanation:** The **countercurrent mechanism** is a sophisticated physiological process involving the Loop of Henle (countercurrent multiplier) and the Vasa Recta (countercurrent exchanger). Its primary purpose is to create and maintain a **hypertonic medullary interstitium**, establishing an osmotic gradient that increases from the cortex (300 mOsm/L) to the deep medulla (up to 1200 mOsm/L). This gradient is essential for the reabsorption of water from the collecting ducts under the influence of ADH, allowing for the excretion of concentrated urine. **Why other options are incorrect:** * **Option A:** Glucose absorption occurs exclusively in the **Proximal Convoluted Tubule (PCT)** via SGLT-2 transporters; it is independent of the medullary gradient. * **Option B:** Renal blood flow is maintained by **autoregulation** (myogenic mechanism and tubuloglomerular feedback), not the countercurrent system. * **Option D:** Uric acid is primarily handled in the PCT through a complex process of filtration, reabsorption, and secretion. **High-Yield NEET-PG Pearls:** * **Countercurrent Multiplier:** The **Loop of Henle** (specifically the thick ascending limb) actively pumps out NaCl, which is the "single effect" that multiplies the gradient. * **Countercurrent Exchanger:** The **Vasa Recta** maintains the gradient by removing excess water and returning solutes to the medulla without washing out the hypertonicity. * **Urea Recycling:** Urea contributes nearly 50% of the medullary osmolarity, especially during dehydration. * **Site of Action:** Loop diuretics (e.g., Furosemide) inhibit the Na-K-2Cl cotransporter in the thick ascending limb, thereby "washing out" this gradient and causing diuresis.

Q: The endothelins are secreted mainly by endothelial cells. All of the following are true regarding endothelins, EXCEPT:

Enhances renal pressure natriuresis. ### Explanation **Endothelins (ET)** are potent peptide vasoconstrictors primarily secreted by damaged or stressed endothelial cells. Understanding their physiological role is crucial for NEET-PG, as they represent a major counter-regulatory system to nitric oxide. **1. Why Option A is the Correct Answer (The Exception):** Endothelins **inhibit** rather than enhance renal pressure natriuresis. Under normal physiological conditions, pressure natriuresis is the mechanism by which the kidney increases sodium excretion in response to elevated blood pressure. Endothelin-1 (ET-1) acts as a powerful antinatriuretic agent by causing profound renal vasoconstriction and decreasing renal blood flow. By reducing the filtered load of sodium and altering peritubular capillary hemodynamics, it opposes the excretion of sodium, thereby contributing to hypertension. **2. Analysis of Other Options:** * **B. Vasoconstriction:** ET-1 is one of the most potent endogenous vasoconstrictors known. It acts via $ET_A$ receptors on vascular smooth muscle to increase intracellular calcium. * **C. Decreased GFR:** By causing intense constriction of both afferent and efferent arterioles (with a preference for the afferent), endothelins significantly reduce renal blood flow and the glomerular filtration rate (GFR). * **D. Cardiac Hypertrophy:** Endothelins act as growth factors for myocytes. Chronic elevation of ET-1 levels is linked to cardiac remodeling, hypertrophy, and the progression of heart failure. **3. High-Yield Clinical Pearls for NEET-PG:** * **Receptors:** $ET_A$ (Vasoconstriction, growth) and $ET_B$ (Vasodilation via NO release, clearance of ET-1). * **Clinical Correlation:** **Bosentan** is a dual $ET_A/ET_B$ receptor antagonist used in the treatment of Pulmonary Arterial Hypertension (PAH). * **Stimuli for Release:** Thrombin, Epinephrine, and Angiotensin II stimulate ET-1 release; Nitric Oxide and ANP inhibit it.

Q: Which of the following is true regarding Glomerular Filtration Rate (GFR)?

Inulin clearance measures the GFR.. **Explanation:** **1. Why Option B is Correct:** Inulin is a fructose polysaccharide that serves as the **gold standard** for measuring GFR. For a substance to measure GFR accurately, it must be freely filtered at the glomerulus and neither reabsorbed nor secreted by the renal tubules. Inulin meets these criteria perfectly; therefore, the amount of inulin filtered equals the amount excreted in the urine (**Clearance of Inulin = GFR**). **2. Why Other Options are Incorrect:** * **Option A:** The normal GFR is approximately **125 mL/min** or **180 L/day**. 625 mL/min is the average Renal Plasma Flow (RPF). * **Option C:** According to Starling’s forces, GFR = $K_f \times [(P_{gc} - P_{bs}) - (\pi_{gc} - \pi_{bs})]$. **Plasma oncotic pressure ($\pi_{gc}$)** opposes filtration. Therefore, an increase in plasma oncotic pressure (e.g., dehydration) **decreases** the GFR. * **Option D:** While early-stage diabetes can cause "hyperfiltration," chronic Diabetes Mellitus leads to diabetic nephropathy, thickening of the basement membrane, and loss of surface area, which ultimately **decreases** the GFR. **3. High-Yield Clinical Pearls for NEET-PG:** * **Creatinine Clearance:** Used clinically to estimate GFR. It slightly **overestimates** GFR because a small amount of creatinine is secreted by the tubules. * **Para-aminohippuric acid (PAH) Clearance:** Used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and almost completely secreted. * **Filtration Fraction (FF):** GFR / RPF (Normal $\approx$ 0.2 or 20%). * **Most sensitive indicator** of early renal failure is a decrease in GFR.

Q: In a given scenario, the filtered urea load is 100 mg/min and the urinary excretion of urea is 25 mg/min. What is the net urea reabsorption rate?

75 mg/min. ***75 mg/min*** - **Urea** is freely filtered at the glomerulus with a filtered load of approximately **100 mg/min**, and only **25 mg/min** is excreted in urine. - Net reabsorption is calculated as **filtered load minus excretion** (100 - 25 = 75 mg/min), representing the amount reabsorbed by the nephron. *0 mg/min* - This would imply **no urea reabsorption** occurs in the nephron, which contradicts normal renal physiology. - Urea undergoes significant reabsorption in the **proximal tubule** (~50%) and **inner medullary collecting duct**. *25 mg/min* - This represents the **excretion rate** of urea in urine, not the reabsorption rate. - Confusing excretion with reabsorption ignores the **75 mg/min** that is actively reclaimed by the kidney. *50 mg/min* - This only accounts for urea reabsorption in the **proximal tubule** alone, ignoring other nephron segments. - Additional reabsorption occurs in the **inner medullary collecting duct** via **UT-A1** and **UT-A3** transporters, contributing to the total 75 mg/min.

Q: Which of the following is NOT a cause for polyuria?

Hypocalcaemia. ### Explanation The correct answer is **D. Hypocalcaemia**. In renal physiology, **Hypercalcemia** (not hypocalcaemia) is a well-known cause of polyuria. #### Why Hypocalcaemia is the Correct Answer: Hypocalcaemia does not cause polyuria. In contrast, **Hypercalcemia** causes polyuria through two primary mechanisms: 1. **Nephrogenic Diabetes Insipidus:** High calcium levels inhibit the action of ADH (Vasopressin) on the V2 receptors in the collecting ducts. 2. **Direct Inhibition:** It inhibits the Na-K-2Cl cotransporter in the thick ascending limb, leading to natriuresis and loss of concentrating ability. #### Analysis of Incorrect Options: * **A. Excess fluid intake:** Leads to **Primary Polydipsia**. The increased water load suppresses ADH secretion from the posterior pituitary, resulting in the excretion of large volumes of dilute urine (water diuresis). * **B. Hyperglycemia:** Causes **Osmotic Diuresis**. When blood glucose exceeds the renal threshold (~180 mg/dL), glucose remains in the tubular lumen. It acts as an osmotically active particle, dragging water with it and preventing reabsorption. * **C. Hypokalemia:** Chronic potassium depletion causes functional and structural changes in the renal tubules, leading to **Nephrogenic Diabetes Insipidus** by interfering with ADH-induced cAMP production. #### NEET-PG High-Yield Pearls: * **Definition of Polyuria:** Urine output >3 Liters/day in adults. * **Electrolyte triggers for Polyuria:** Remember the "Highs and Lows"—**High** Calcium and **Low** Potassium. * **Lithium:** The most common drug causing drug-induced Nephrogenic Diabetes Insipidus. * **Water Deprivation Test:** Used to differentiate between Central DI, Nephrogenic DI, and Primary Polydipsia.

Question 1

Renal plasma flow is best determined by:

Accepted Answer

PAH

Answer

Inulin

Answer

Creatinine

Answer

Mannitol

Question 2

Which hormone is secreted by the kidney?

Accepted Answer

Erythropoietin

Answer

Angiotensin I

Answer

Angiotensin II

Answer

Thrombomodulin

Question 3

Stretch reflex of the urinary bladder is integrated at which level of the spinal cord?

Accepted Answer

Sacral portion of the spinal cord

Answer

Lumbar portion of the spinal cord

Answer

Substantia gelatinosa

Answer

Sympathetic plexus

Question 4

The countercurrent mechanism in the kidney is primarily responsible for which of the following?

Accepted Answer

Creation of an osmotic gradient in the medulla

Answer

Absorption of glucose

Answer

Maintenance of blood flow

Answer

Secretion of uric acid

Question 5

Low and fixed specific gravity of urine is seen in which of the following conditions?

Accepted Answer

Chronic renal failure

Answer

Diabetes mellitus

Answer

Diabetes insipidus

Answer

Acute glomerulonephritis

Question 6

Which substance is minimally filtered through the glomerulus?

Accepted Answer

Myoglobin

Answer

Glucose

Answer

Inulin

Answer

Creatinine

Question 7

The endothelins are secreted mainly by endothelial cells. All of the following are true regarding endothelins, EXCEPT:

Accepted Answer

Enhances renal pressure natriuresis

Answer

Vasoconstriction

Answer

Decreased GFR

Answer

Act on the heart to cause hypertrophy

Question 8

Which of the following is true regarding Glomerular Filtration Rate (GFR)?

Accepted Answer

Inulin clearance measures the GFR.

Answer

Normal glomerular filtration rate is 625 L/day.

Answer

Increased plasma oncotic pressure increases the GFR.

Answer

Diabetes mellitus increases the GFR.

Question 9

In a given scenario, the filtered urea load is 100 mg/min and the urinary excretion of urea is 25 mg/min. What is the net urea reabsorption rate?

Accepted Answer

75 mg/min

Answer

0 mg/min

Answer

25 mg/min

Answer

50 mg/min

Question 10

Which of the following is NOT a cause for polyuria?

Accepted Answer

Hypocalcaemia

Answer

Excess fluid intake

Answer

Hyperglycemia

Answer

Hypokalemia

Renal Physiology — MCQs

Renal Physiology — MCQs

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