Renal Physiology Practice Questions

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.

Q: In renal glycosuria, what is the renal threshold for glucose?

Low. **Explanation:** In a healthy individual, glucose is freely filtered at the glomerulus and almost entirely reabsorbed in the proximal convoluted tubule (PCT) via SGLT2 and SGLT1 transporters. The **Renal Threshold** is the plasma glucose concentration at which the transporters become saturated, and glucose begins to appear in the urine (glycosuria). In normal adults, this threshold is approximately **180 mg/dL**. **1. Why "Low" is correct:** **Renal Glycosuria** (also known as Benign Glycosuria) is a condition where glucose is excreted in the urine despite **normal** blood glucose levels. This occurs because of a functional defect in the SGLT2 transporters or a reduced affinity for glucose. Because the "spillover" point into the urine happens at a much lower plasma concentration than normal (e.g., 120 mg/dL instead of 180 mg/dL), the renal threshold is considered **Low**. **2. Why other options are wrong:** * **High / Greatly Increased:** A high threshold would mean the kidney is "better" at retaining glucose, which occurs in conditions like chronic kidney disease (CKD) or aging, where the GFR decreases. * **Same:** If the threshold were the same, glycosuria would only occur during hyperglycemia (Diabetes Mellitus), not in renal glycosuria. **High-Yield Clinical Pearls for NEET-PG:** * **Transport Maximum ($T_m$):** The maximum rate of glucose reabsorption, normally **375 mg/min** in men and **303 mg/min** in women. * **Splay:** The difference between the actual renal threshold and the theoretical $T_m$ (due to nephron heterogeneity). * **SGLT2 Inhibitors (e.g., Dapagliflozin):** These drugs pharmacologically **lower the renal threshold** to treat Diabetes Mellitus by inducing glycosuria. * **Fanconi Syndrome:** A generalized PCT defect where a low renal threshold for glucose is seen alongside aminoaciduria, phosphaturia, and bicarbonaturia.

Q: A hypertensive patient is found to have a partial obstruction of the renal artery due to an atherosclerotic plaque. The resultant decrease in blood flow causes increased release of an enzyme from which of the following structures?

Juxtaglomerular cells. **Explanation:** The correct answer is **C. Juxtaglomerular cells.** **Mechanism:** This scenario describes **Renovascular Hypertension**. A partial obstruction of the renal artery (e.g., due to atherosclerosis) leads to a decrease in renal perfusion pressure. This drop in pressure is sensed by the **intrarenal baroreceptors** located in the **Juxtaglomerular (JG) cells**, which are specialized modified smooth muscle cells found primarily in the walls of the **afferent arterioles**. In response to decreased stretch (hypotension) or sympathetic stimulation, these cells secrete the enzyme **Renin**. Renin initiates the Renin-Angiotensin-Aldosterone System (RAAS), leading to systemic vasoconstriction and sodium retention to restore blood pressure. **Analysis of Incorrect Options:** * **A. Afferent arterioles:** While JG cells are located *within* the walls of the afferent arterioles, the specific structure responsible for enzyme (Renin) secretion is the JG cell itself. In NEET-PG, always choose the most specific anatomical structure provided. * **B. Arcuate arteries:** These are conduit vessels located at the corticomedullary junction. They do not possess specialized secretory functions for blood pressure regulation. * **D. Kupffer cells:** These are specialized macrophages located in the **liver** sinusoids. They are part of the reticuloendothelial system and are not involved in the RAAS. **High-Yield Clinical Pearls for NEET-PG:** * **Goldblatt Kidney:** This clinical scenario is the human equivalent of the "one-kidney, one-clip" Goldblatt model of hypertension. * **Stimuli for Renin Release:** 1) Decreased perfusion pressure (baroreceptors), 2) Decreased NaCl delivery to the Macula Densa, and 3) Sympathetic stimulation (Beta-1 receptors). * **Histology:** JG cells contain prorenin granules and are considered the "endocrine" component of the Juxtaglomerular Apparatus (JGA).

Q: Which aquaporin channel is primarily mediated through ADH action?

Aquaporin 2. **Explanation:** The correct answer is **Aquaporin 2 (AQP2)**. **Why Aquaporin 2 is correct:** Antidiuretic Hormone (ADH), also known as Vasopressin, regulates water reabsorption in the **Principal cells** of the late distal tubule and collecting ducts. When ADH binds to the **V2 receptors** on the basolateral membrane, it triggers a cAMP-mediated signaling pathway. This causes the translocation of intracellular vesicles containing **Aquaporin 2** channels to the **apical (luminal) membrane**. This increases the water permeability of the membrane, allowing water to be reabsorbed into the hypertonic medullary interstitium. **Why other options are incorrect:** * **GLUT:** These are glucose transporters (e.g., GLUT2 in the proximal tubule) and are not involved in water transport. * **Aquaporin 1:** These channels are constitutively active (always open) and are primarily located in the **Proximal Convoluted Tubule (PCT)** and the descending limb of the Loop of Henle. They are **not** regulated by ADH. * **Aquaporin 3 & 4:** These channels are located on the **basolateral membrane** of the collecting duct cells. While they facilitate the exit of water from the cell into the blood, they are constitutively expressed and not the primary site of ADH-mediated regulation. **High-Yield Clinical Pearls for NEET-PG:** * **Diabetes Insipidus (DI):** Central DI is caused by a deficiency of ADH, while Nephrogenic DI is often due to a defect in the V2 receptor or the **AQP2 channel** itself. * **V2 Receptor:** G-protein coupled receptor (Gs) $\rightarrow$ Adenylyl cyclase $\rightarrow$ cAMP $\rightarrow$ Protein Kinase A $\rightarrow$ AQP2 insertion. * **Lithium:** A common cause of drug-induced nephrogenic DI as it inhibits the signaling pathway that leads to AQP2 expression.

Q: All of the following are true about juxtamedullary nephrons except?

It has a slow rate of filtration.. **Explanation:** In the human kidney, there are two types of nephrons: **Cortical nephrons** (85%) and **Juxtamedullary (JM) nephrons** (15%). **Why Option C is the correct (False) statement:** Juxtamedullary nephrons actually have a **higher Glomerular Filtration Rate (GFR)** compared to cortical nephrons. This is due to their larger glomeruli and higher hydrostatic pressure within the glomerular capillaries. Therefore, stating they have a "slow rate of filtration" is physiologically incorrect. **Analysis of other options:** * **Option A:** It is a standard anatomical fact that JM nephrons constitute approximately **15%** of the total nephron population. * **Option B:** JM nephrons are characterized by **long loops of Henle** that extend deep into the renal medulla, reaching the tips of the renal papillae. * **Option D:** Their primary physiological role is the **concentration of urine**. The long loops of Henle, along with the associated **Vasa Recta** (specialized peritubular capillaries), are essential for maintaining the medullary osmotic gradient via the **counter-current mechanism**. **High-Yield Clinical Pearls for NEET-PG:** * **Vasa Recta:** These are only found associated with Juxtamedullary nephrons; cortical nephrons lack them. * **Renin Content:** Juxtamedullary nephrons contain **less renin** than cortical nephrons. * **Function during Stress:** During periods of decreased renal blood flow (like heart failure), blood is shunted from cortical to JM nephrons to maximize water reabsorption.

Question 1

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 2

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 3

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 4

Which of the following is NOT a cause for polyuria?

Accepted Answer

Hypocalcaemia

Answer

Excess fluid intake

Answer

Hyperglycemia

Answer

Hypokalemia

Question 5

In renal glycosuria, what is the renal threshold for glucose?

Accepted Answer

Low

Answer

High

Answer

Same

Answer

Greatly increased

Question 6

What is the effect of sympathetic stimulation on Glomerular Filtration Rate (GFR)?

Accepted Answer

Decreased GFR with afferent arteriolar constriction

Answer

Increased GFR with afferent arteriolar dilatation

Answer

Increased GFR with efferent arteriolar constriction

Answer

Decreased GFR with efferent arteriolar dilatation

Question 7

A hypertensive patient is found to have a partial obstruction of the renal artery due to an atherosclerotic plaque. The resultant decrease in blood flow causes increased release of an enzyme from which of the following structures?

Accepted Answer

Juxtaglomerular cells

Answer

Afferent arterioles

Answer

Arcuate arteries

Answer

Kupffer cells

Question 8

Which aquaporin channel is primarily mediated through ADH action?

Accepted Answer

Aquaporin 2

Answer

GLUT

Answer

Aquaporin 1

Answer

Aquaporin 3

Question 9

Atrial Natriuretic Peptide (ANP) causes what change?

Accepted Answer

Increase in blood volume

Answer

Increase in cardiac output

Answer

Increase in urine output

Answer

Decrease in renin secretion

Question 10

All of the following are true about juxtamedullary nephrons except?

Accepted Answer

It has a slow rate of filtration.

Answer

It forms 15% of total nephrons.

Answer

It has a long loop of Henle.

Answer

Major function is concentration of urine by counter-current mechanism.

Renal Physiology — MCQs

Renal Physiology — MCQs

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