Renal Physiology Practice Questions

Q: All of the following cause relaxation of mesangial cells except?

PGF2. **Explanation:** The glomerular filtration rate (GFR) is significantly influenced by the surface area available for filtration, which is regulated by the contraction and relaxation of **mesangial cells**. These cells contain myofilaments and respond to various vasoactive substances. **Why PGF2 is the correct answer:** Mesangial cells contract in response to substances that increase intracellular calcium or specific prostaglandins. **Prostaglandin F2α (PGF2α)**, along with Thromboxane A2, Endothelin, Angiotensin II, and Vasopressin (V1 receptors), acts as a **potent stimulator of mesangial cell contraction**. Contraction reduces the effective capillary surface area, thereby decreasing the GFR. **Analysis of Incorrect Options:** * **CAMP (Cyclic AMP):** Intracellular signaling via cAMP generally leads to the relaxation of smooth muscle-like cells. Agents that increase cAMP levels in mesangial cells promote relaxation. * **Dopamine:** Dopamine acts as a vasodilator in the renal vasculature and induces mesangial cell relaxation, helping to maintain or increase renal blood flow and GFR. * **ANP (Atrial Natriuretic Peptide):** ANP is a potent relaxant of mesangial cells. By relaxing these cells, ANP increases the effective filtration surface area, which contributes to its natriuretic effect by increasing GFR. **High-Yield Facts for NEET-PG:** * **Relaxants (Increase GFR):** ANP, Dopamine, cAMP, PGE2, and Nitric Oxide (NO). * **Contractors (Decrease GFR):** Angiotensin II (most potent), Vasopressin, Endothelin, PGF2α, and Thromboxane A2. * **Clinical Pearl:** Angiotensin II preferentially constricts the efferent arteriole (increasing GFR) but simultaneously contracts mesangial cells (decreasing surface area). The net effect on GFR depends on the balance between these two actions.

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: 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.

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.

Question 1

All of the following cause relaxation of mesangial cells except?

Accepted Answer

PGF2

Answer

CAMP

Answer

Dopamine

Answer

ANP

Question 2

Renal plasma flow is best determined by:

Accepted Answer

PAH

Answer

Inulin

Answer

Creatinine

Answer

Mannitol

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

Which substance is minimally filtered through the glomerulus?

Accepted Answer

Myoglobin

Answer

Glucose

Answer

Inulin

Answer

Creatinine

Question 6

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 7

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 8

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 9

Which of the following is NOT a cause for polyuria?

Accepted Answer

Hypocalcaemia

Answer

Excess fluid intake

Answer

Hyperglycemia

Answer

Hypokalemia

Question 10

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

Accepted Answer

Low

Answer

High

Answer

Same

Answer

Greatly increased

Renal Physiology — MCQs

Renal Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?