Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 341: Renal plasma flow is best determined by:

A. Inulin
B. Creatinine
C. PAH (Correct Answer)
D. Mannitol

Explanation: **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.

Question 342: Which hormone is secreted by the kidney?

A. Erythropoietin (Correct Answer)
B. Angiotensin I
C. Angiotensin II
D. Thrombomodulin

Explanation: **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.

Question 343: Stretch reflex of the urinary bladder is integrated at which level of the spinal cord?

A. Sacral portion of the spinal cord (Correct Answer)
B. Lumbar portion of the spinal cord
C. Substantia gelatinosa
D. Sympathetic plexus

Explanation: **Explanation:** The stretch reflex of the urinary bladder, also known as the **Micturition Reflex**, is an autonomic spinal cord reflex. When the bladder fills (approx. 300–400 mL), stretch receptors in the bladder wall (detrusor muscle) send sensory signals via the **pelvic nerves** to the **sacral segments (S2, S3, and S4)** of the spinal cord. This area is known as the **Sacral Micturition Center**. The reflex is integrated here, and parasympathetic motor impulses are sent back via the pelvic nerves to cause detrusor contraction and internal sphincter relaxation, leading to voiding. **Analysis of Options:** * **Option A (Correct):** The parasympathetic supply (the primary motor driver for micturition) originates from the intermediolateral columns of **S2-S4**. * **Option B:** The lumbar portion (L1-L2) houses the sympathetic center. Sympathetic nerves (Hypogastric nerve) promote bladder filling by relaxing the detrusor and contracting the internal sphincter; they do not mediate the acute stretch/voiding reflex. * **Option C:** Substantia gelatinosa is located in the dorsal horn of the spinal cord and is primarily involved in modulating pain signals (Gate Control Theory), not autonomic reflexes. * **Option D:** While plexuses exist, the integration of a reflex arc requires a central nervous system component (the spinal cord). **High-Yield Clinical Pearls for NEET-PG:** 1. **Higher Centers:** While the reflex is integrated at the sacral cord, it is modulated by the **Pontine Micturition Center (Barrington’s nucleus)** and inhibited by the cerebral cortex. 2. **Nerve Supply:** * **Pelvic Nerve (S2-S4):** Parasympathetic (Emptying). * **Hypogastric Nerve (T12-L2):** Sympathetic (Filling). * **Pudendal Nerve (S2-S4):** Somatic (Voluntary control of external sphincter). 3. **Lesions:** A lesion at the sacral level results in an **Atonic/Non-reflexic bladder**, whereas a lesion above the sacral level (but below the pons) results in an **Automatic/Spastic bladder**.

Question 344: The countercurrent mechanism in the kidney is primarily responsible for which of the following?

A. Absorption of glucose
B. Maintenance of blood flow
C. Creation of an osmotic gradient in the medulla (Correct Answer)
D. Secretion of uric acid

Explanation: **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.

Question 345: Low and fixed specific gravity of urine is seen in which of the following conditions?

A. Diabetes mellitus
B. Diabetes insipidus
C. Chronic renal failure (Correct Answer)
D. Acute glomerulonephritis

Explanation: **Explanation:** The correct answer is **Chronic Renal Failure (CRF)**. This phenomenon is known as **Isosthenuria**, where the kidney loses its ability to either concentrate or dilute urine. **1. Why Chronic Renal Failure is correct:** In advanced CRF, there is a progressive loss of functioning nephrons. The remaining "survivor" nephrons undergo compensatory hypertrophy and experience an increased solute load per nephron (osmotic diuresis). This leads to the destruction of the medullary osmotic gradient and a lack of responsiveness to ADH. Consequently, the kidney cannot modify the glomerular filtrate, and the urine excreted has the same osmolality and specific gravity as the protein-free plasma (approx. **300 mOsm/L** or a specific gravity of **1.010**). This is "fixed" because it does not change regardless of fluid intake. **2. Why other options are incorrect:** * **Diabetes Mellitus:** Characterized by **high specific gravity** (>1.030) due to glycosuria. The presence of glucose (a large molecule) increases the density of urine despite the increased volume (osmotic diuresis). * **Diabetes Insipidus:** Characterized by **low specific gravity** (<1.005) because of a lack of ADH (Central) or resistance to it (Nephrogenic). However, it is not "fixed"; it is persistently dilute. * **Acute Glomerulonephritis:** Usually presents with oliguria and **high specific gravity** as the kidneys attempt to conserve water, and the tubular function is often initially preserved compared to the glomerular filtration rate. **High-Yield Clinical Pearls for NEET-PG:** * **Isosthenuria:** Specific gravity fixed at **1.010**. It is a hallmark of end-stage renal disease. * **Hyposthenuria:** Specific gravity **<1.010** (seen in Diabetes Insipidus). * **Hypersthenuria:** Specific gravity **>1.010** (seen in DM, SIADH, or dehydration). * The most reliable test for renal concentrating capacity is the **Urine Osmolality test**, but specific gravity is a common bedside surrogate.

Question 346: Which substance is minimally filtered through the glomerulus?

A. Glucose
B. Inulin
C. Creatinine
D. Myoglobin (Correct Answer)

Explanation: **Explanation:** The filtration of substances across the glomerular filtration barrier (GFB) is determined by two primary factors: **molecular size (radius)** and **electrical charge**. The GFB consists of fenestrated endothelium, the basement membrane (rich in negatively charged heparan sulfate), and podocyte slit diaphragms. **Why Myoglobin is the Correct Answer:** Glomerular permeability is inversely proportional to molecular weight. Small solutes like electrolytes and glucose have a filtration ratio of 1.0 (freely filtered). As molecular weight increases, filtration decreases. * **Myoglobin** is a protein with a molecular weight of approximately **17,000 Da**. While it is smaller than albumin (69,000 Da), it is significantly larger than glucose or inulin. Its size and globular structure result in a much lower filtration clearance compared to the other options listed. **Analysis of Incorrect Options:** * **A. Glucose (180 Da):** A small, uncharged molecule that is **freely filtered** at the glomerulus. It is normally 100% reabsorbed in the proximal convoluted tubule. * **B. Inulin (5,000 Da):** A fructose polymer used to measure GFR. It is **freely filtered** and is neither reabsorbed nor secreted by the tubules. * **C. Creatinine (113 Da):** A small metabolic byproduct that is **freely filtered**. (Note: A small amount is also secreted, making it a slight overestimate of GFR). **High-Yield Clinical Pearls for NEET-PG:** * **Charge Selectivity:** The GFB is negatively charged. Therefore, for molecules of the same size, **cations** (positive) are filtered most easily, followed by neutral molecules, while **anions** (negative) are filtered least. * **Clinical Correlation:** In **Rhabdomyolysis**, massive amounts of myoglobin are released into the blood. Although "minimally filtered" compared to glucose, the sheer volume filtered can exceed the kidney's handling capacity, leading to **Acute Tubular Necrosis (ATN)** due to direct toxicity and cast formation. * **Albumin:** Has a molecular weight of 69,000 Da and is negatively charged; its filtration ratio is nearly zero (0.005).

Question 347: The endothelins are secreted mainly by endothelial cells. All of the following are true regarding endothelins, EXCEPT:

A. Enhances renal pressure natriuresis (Correct Answer)
B. Vasoconstriction
C. Decreased GFR
D. Act on the heart to cause hypertrophy

Explanation: ### 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.

Question 348: Which of the following is true regarding Glomerular Filtration Rate (GFR)?

A. Normal glomerular filtration rate is 625 L/day.
B. Inulin clearance measures the GFR. (Correct Answer)
C. Increased plasma oncotic pressure increases the GFR.
D. Diabetes mellitus increases the GFR.

Explanation: **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.

Question 349: 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?

A. 0 mg/min
B. 25 mg/min
C. 50 mg/min
D. 75 mg/min (Correct Answer)

Explanation: ***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.

Question 350: Which of the following is NOT a cause for polyuria?

A. Excess fluid intake
B. Hyperglycemia
C. Hypokalemia
D. Hypocalcaemia (Correct Answer)

Explanation: ### 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.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

Tubular Reabsorption and Secretion

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Concentration and Dilution of Urine

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Acid-Base Regulation by the Kidneys

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Sodium and Water Balance

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Potassium Regulation

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Calcium and Phosphate Handling

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Micturition Physiology

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Renal Function Tests

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Integrative Responses to Fluid Challenges

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