Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 241: Glomerular filtration rate (GFR) increases if which of the following occurs?

A. Afferent arteriole constricts
B. Afferent arteriole dilates (Correct Answer)
C. Efferent arteriole dilates
D. None of the above

Explanation: **Explanation:** The Glomerular Filtration Rate (GFR) is primarily determined by the **Glomerular Hydrostatic Pressure ($P_{GC}$)**. This pressure is regulated by the relative resistance of the afferent and efferent arterioles. **1. Why Option B is Correct:** When the **afferent arteriole dilates**, resistance to blood flow entering the glomerulus decreases. This leads to an increase in renal blood flow (RBF) and a subsequent rise in glomerular hydrostatic pressure. According to Starling’s forces, an increase in $P_{GC}$ directly enhances the filtration of plasma across the basement membrane, thereby **increasing GFR**. **2. Why Incorrect Options are Wrong:** * **Afferent arteriole constricts (Option A):** Constriction increases resistance *before* the glomerulus, reducing the blood flow and lowering the $P_{GC}$. This results in a **decrease in GFR**. * **Efferent arteriole dilates (Option C):** Dilation of the efferent arteriole allows blood to leave the glomerular capillaries more easily (decreased "back pressure"). This lowers the $P_{GC}$ and consequently **decreases GFR**. (Note: Conversely, *constriction* of the efferent arteriole increases GFR, provided the constriction is not so severe that it severely limits RBF). **Clinical Pearls for NEET-PG:** * **ANP/BNP:** These peptides increase GFR by simultaneously dilating the afferent and constricting the efferent arterioles. * **NSAIDs:** These inhibit prostaglandins (which normally dilate the afferent arteriole), leading to afferent constriction and a drop in GFR (Acute Kidney Injury). * **ACE Inhibitors:** These prevent the production of Angiotensin II (which normally constricts the efferent arteriole), leading to efferent dilation and a potential drop in GFR.

Question 242: Which of the following formulas denotes free water clearance?

A. Urine flow rate multiplied by osmolal clearance
B. Urine flow rate minus osmolal clearance (Correct Answer)
C. Urine flow rate divided by osmolal clearance
D. Urine flow rate plus osmolal clearance

Explanation: ### Explanation **Concept Overview** Free water clearance ($C_{H_2O}$) is a measure of the kidney's ability to dilute or concentrate urine. It represents the volume of blood plasma that is cleared of solute-free water per unit time. Conceptually, total urine flow ($V$) is composed of two parts: 1. **Osmolal Clearance ($C_{osm}$):** The volume of water required to excrete solutes at a concentration iso-osmotic to plasma. 2. **Free Water Clearance ($C_{H_2O}$):** The volume of "pure" water added to or removed from the iso-osmotic volume to produce the final urine concentration. **Why Option B is Correct** The relationship is expressed by the formula: **$V = C_{osm} + C_{H_2O}$**. By rearranging this formula to solve for free water clearance, we get: **$C_{H_2O} = V - C_{osm}$** Where $V$ is the urine flow rate and $C_{osm}$ is the osmolal clearance (calculated as $\frac{U_{osm} \times V}{P_{osm}}$). **Analysis of Incorrect Options** * **Option A & C:** Multiplication and division do not represent the physiological relationship between total volume and its constituent parts (solute-bound vs. solute-free). * **Option D:** Adding osmolal clearance to urine flow rate is mathematically incorrect; $V$ is the total sum, not a component to be added to $C_{osm}$. **High-Yield Clinical Pearls for NEET-PG** * **Positive $C_{H_2O}$ ($V > C_{osm}$):** Occurs when urine is dilute (hypo-osmotic to plasma), such as in **Diabetes Insipidus** or excessive water intake. * **Negative $C_{H_2O}$ ($V < C_{osm}$):** Also called "free water reabsorption" ($T^c_{H_2O}$). Occurs when urine is concentrated (hyper-osmotic to plasma) due to high **ADH** levels (e.g., SIADH or dehydration). * **Zero $C_{H_2O}$:** Occurs when urine is iso-osmotic to plasma (e.g., action of loop diuretics which impair the kidney's ability to dilute or concentrate urine).

Question 243: What is the main driving force for water reabsorption by the proximal tubule epithelium?

A. Active reabsorption of amino acids and glucose
B. Active reabsorption of Na+ (Correct Answer)
C. Active reabsorption of water
D. Pinocytosis

Explanation: **Explanation:** The proximal convoluted tubule (PCT) is responsible for reabsorbing approximately 65% of the filtered load of water and electrolytes. The fundamental driving force for this process is the **active reabsorption of Na+**. 1. **Mechanism (Why B is correct):** The Na+/K+ ATPase pump located on the basolateral membrane of the tubular cells actively pumps Na+ out of the cell into the interstitium. This creates a low intracellular Na+ concentration and a negative intracellular potential, establishing a strong electrochemical gradient. This gradient drives Na+ from the tubular lumen into the cell. As Na+ is reabsorbed, it creates an osmotic gradient that "pulls" water molecules across the epithelium (via aquaporin-1 channels and paracellular pathways) to maintain osmotic equilibrium. This is known as **obligatory water reabsorption**. 2. **Analysis of Incorrect Options:** * **Option A:** While glucose and amino acids are reabsorbed via secondary active transport, they are co-transported *with* Na+. Their contribution to the total osmotic gradient is significantly smaller than that of Na+ itself. * **Option C:** Water is always reabsorbed **passively** along osmotic gradients in the kidney; there is no such mechanism as the "active" transport of water molecules. * **Option D:** Pinocytosis is used for the reabsorption of small amounts of filtered proteins, not for bulk water movement. **High-Yield NEET-PG Pearls:** * **Isotonic Reabsorption:** In the PCT, water follows solutes so proportionately that the tubular fluid remains **iso-osmotic** to plasma (approx. 300 mOsm/L). * **SGLT-2 Inhibitors:** Drugs like Dapagliflozin act in the PCT by inhibiting Na+/Glucose co-transport, leading to glucosuria and osmotic diuresis. * **Carbonic Anhydrase Inhibitors:** Acetazolamide acts here, inhibiting HCO3- and Na+ reabsorption, which also decreases water reabsorption.

Question 244: The countercurrent mechanism is responsible for concentrating urine. Which of the following statements regarding this mechanism is FALSE?

A. It involves countercurrent flow in the vasa recta and the loop of Henle.
B. It is primarily seen in juxtamedullary nephrons.
C. It is crucial for creating and maintaining the medullary osmotic gradient.
D. Urea plays a significant role in establishing this gradient. (Correct Answer)

Explanation: The countercurrent mechanism is a vital physiological process that allows the kidneys to excrete hypertonic urine. **Explanation of the Correct Answer:** The statement **"Urea plays a significant role in establishing this gradient"** is technically **FALSE** in the context of the initial *establishment* of the gradient. While urea is essential for **maintaining** and enhancing the medullary osmotic gradient (contributing up to 50% of the osmolarity in the inner medulla via urea recycling), the gradient is primarily **established** by the active transport of Sodium and Chloride ions out of the Thick Ascending Limb (TAL) of the Loop of Henle. This distinction between "establishment" (NaCl) and "maintenance/contribution" (Urea) is a frequent high-yield nuance in renal physiology. **Analysis of Other Options:** * **Option A:** Correct. The mechanism consists of two parts: the **Countercurrent Multiplier** (Loop of Henle) which creates the gradient, and the **Countercurrent Exchanger** (Vasa Recta) which preserves it. * **Option B:** Correct. Juxtamedullary nephrons have long Loops of Henle that descend deep into the renal medulla, making them the primary drivers of the concentration gradient. * **Option C:** Correct. The fundamental purpose of the countercurrent system is to create a hypertonic medullary interstitium, which allows for water reabsorption from the collecting ducts under the influence of ADH. **High-Yield NEET-PG Pearls:** * **The Single Effect:** The active transport of NaCl out of the TAL into the interstitium, creating a 200 mOsm/L gradient. * **Vasa Recta:** Acts as an exchanger; its slow blood flow prevents the "washout" of the medullary gradient. * **ADH (Vasopressin):** Increases the permeability of the medullary collecting duct to urea, facilitating urea recycling and further concentrating the urine.

Question 245: Glomerular filtration rate (GFR) is increased by which of the following hormones?

A. Atrial Natriuretic Peptide (ANP) (Correct Answer)
B. Vasopressin
C. Histamine
D. Endothelin

Explanation: **Explanation:** The Glomerular Filtration Rate (GFR) is primarily determined by the net filtration pressure and the capillary filtration coefficient ($K_f$). **Why Atrial Natriuretic Peptide (ANP) is Correct:** ANP is secreted by the cardiac atria in response to stretch (volume overload). It increases GFR through a dual mechanism: 1. **Afferent/Efferent Modulation:** It causes **vasodilation of the afferent arteriole** and **vasoconstriction of the efferent arteriole**. This "push-pull" effect significantly increases the glomerular hydrostatic pressure. 2. **Surface Area:** It relaxes glomerular mesangial cells, increasing the effective surface area available for filtration ($K_f$). **Analysis of Incorrect Options:** * **Vasopressin (ADH):** Its primary role is water reabsorption in the collecting ducts via V2 receptors. While it can cause systemic vasoconstriction at high doses (V1 receptors), it generally does not increase GFR; in states of dehydration, GFR may actually decrease. * **Histamine:** While histamine is a vasodilator, in the renal microvasculature, it is not a primary physiological regulator of GFR increase compared to ANP. * **Endothelin:** This is a potent **vasoconstrictor**. It causes profound constriction of both afferent and efferent arterioles (with a preference for afferent), leading to a significant **decrease** in GFR. **NEET-PG High-Yield Pearls:** * **Vasoconstrictors that decrease GFR:** Noradrenaline, Adrenaline, Endothelin, and Angiotensin II (though Angiotensin II preferentially constricts the efferent arteriole to maintain GFR during low perfusion). * **Vasodilators that increase GFR:** ANP, Prostaglandins ($PGE_2, PGI_2$), Nitric Oxide (NO), and Bradykinin. * **Clinical Note:** NSAIDs decrease GFR by inhibiting Prostaglandins, which normally keep the afferent arteriole open.

Question 246: What is the best test for Glomerular Filtration Rate (GFR)?

A. Inulin clearance (Correct Answer)
B. Hippuric acid clearance
C. Creatinine clearance
D. Para-aminohippuric acid (PAH) clearance

Explanation: **Explanation:** The **Glomerular Filtration Rate (GFR)** is the volume of fluid filtered from the renal glomerular capillaries into the Bowman's capsule per unit time. To measure GFR accurately, a substance must be **freely filtered** at the glomerulus and must **not be reabsorbed, secreted, or metabolized** by the renal tubules. **Why Inulin Clearance is the Gold Standard:** Inulin, a plant-derived polysaccharide, perfectly meets these criteria. Since every molecule of inulin that enters the nephron does so only via filtration and remains in the tubule until excretion, its clearance rate is exactly equal to the GFR. **Analysis of Other Options:** * **Hippuric acid clearance:** Not used for GFR; it is sometimes used to estimate renal blood flow but is less accurate than PAH. * **Creatinine clearance:** This is the most common **clinical** method to estimate GFR. However, it slightly **overestimates** GFR because a small amount of creatinine is actively secreted by the proximal tubules. * **Para-aminohippuric acid (PAH) clearance:** PAH is both filtered and almost completely secreted by the tubules. Therefore, its clearance is used to measure **Effective Renal Plasma Flow (ERPF)**, not GFR. **High-Yield NEET-PG Pearls:** * **Gold Standard for GFR:** Inulin clearance. * **Most Common Clinical Test for GFR:** Creatinine clearance (Cockcroft-Gault formula). * **Marker for Renal Plasma Flow:** PAH clearance. * **Filtration Fraction (FF):** GFR / Renal Plasma Flow (Normal ≈ 20%). * **Radioisotopes:** I-125 iothalamate is a modern alternative to inulin for precise GFR measurement.

Question 247: How much urine does the average adult pass each day?

A. 1 gallon
B. 1 1/2 Quarts (Correct Answer)
C. 1 1/2 gallons
D. 2 pints

Explanation: **Explanation:** The average adult human produces approximately **1.5 liters (1500 mL)** of urine per day. In the imperial system, 1 liter is roughly equivalent to 1 quart (1.05 quarts). Therefore, **1 ½ quarts** (approximately 1420 mL) is the most accurate representation of normal daily urine output. This volume is the result of renal processing where the kidneys filter about 180 liters of plasma daily, but reabsorb over 99% of the filtrate. The final urine volume depends on hydration status, solute load, and the action of Antidiuretic Hormone (ADH). **Analysis of Options:** * **A & C (1 gallon / 1 ½ gallons):** These represent 3.7 to 5.6 liters. Such high volumes are pathological and indicative of **polyuria**, commonly seen in Diabetes Mellitus or Diabetes Insipidus. * **D (2 pints):** Two pints equal approximately 950 mL. While this is within the broad physiological range, it is lower than the standard "average" used in medical literature for a healthy adult. **NEET-PG High-Yield Pearls:** * **Oliguria:** Defined as urine output **<400 mL/day** in adults. This is the minimum volume required to excrete the daily solute load (obligatory urine volume). * **Anuria:** Defined as **<100 mL/day**. * **Polyuria:** Defined as **>3 Liters/day**. * **Normal GFR:** 125 mL/min (180 L/day). * **Minimum Facultative Reabsorption:** Occurs in the Proximal Convoluted Tubule (PCT), where ~65% of water is reabsorbed isosmotically, regardless of ADH levels.

Question 248: Which of the following substances is neither reabsorbed nor secreted by the renal tubules after glomerular filtration?

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

Explanation: ### Explanation **Correct Option: A (Inulin)** Inulin is a fructose polymer that serves as the **gold standard** for measuring the Glomerular Filtration Rate (GFR). Its unique physiological property is that it is freely filtered at the glomerulus but is **neither reabsorbed nor secreted** by the renal tubules. Consequently, the amount of inulin filtered is exactly equal to the amount excreted in the urine. This allows for the calculation of GFR using the formula: $GFR = \frac{U_{In} \times V}{P_{In}}$. **Analysis of Incorrect Options:** * **B. Creatinine:** While often used clinically to estimate GFR, it is **slightly secreted** by the proximal tubules. This leads to an overestimation of the actual GFR by about 10-20%. * **C. Glucose:** Under normal physiological conditions, glucose is freely filtered but **completely reabsorbed** in the proximal convoluted tubule (PCT) via SGLT-2 transporters. It only appears in urine if the blood glucose exceeds the renal threshold (approx. 180 mg/dL). * **D. Water:** Approximately **99% of filtered water is reabsorbed** throughout the nephron (primarily in the PCT and descending limb of Henle's loop) to maintain fluid balance. **High-Yield NEET-PG Pearls:** * **Para-aminohippuric acid (PAH):** It is filtered and **completely secreted** (at low doses), making it the gold standard for measuring **Renal Plasma Flow (RPF)**. * **Fractional Excretion:** If the clearance of a substance is less than Inulin clearance, the substance undergoes net reabsorption. If it is greater than Inulin clearance, it undergoes net secretion. * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** These drugs block glucose reabsorption in the PCT and are high-yield topics for both Physiology and Pharmacology.

Question 249: What is the effect of efferent arteriole constriction and afferent arteriole dilatation on Glomerular Filtration Rate (GFR)?

A. Increases (Correct Answer)
B. Decreases
C. No change
D. First increases then decreases

Explanation: ### Explanation The Glomerular Filtration Rate (GFR) is primarily determined by the **Glomerular Hydrostatic Pressure ($P_{GC}$)**. This pressure is regulated by the relative resistances of the afferent and efferent arterioles. 1. **Afferent Arteriole Dilatation:** By widening the "inlet," resistance decreases, allowing more blood to flow into the glomerular capillaries. This directly increases $P_{GC}$. 2. **Efferent Arteriole Constriction:** By narrowing the "outlet," blood is "backed up" within the glomerular capillaries. This further elevates the $P_{GC}$. Since both actions (opening the inlet and narrowing the outlet) work synergistically to increase the hydrostatic pressure pushing fluid into Bowman’s space, the net result is a significant **increase in GFR**. #### Analysis of Incorrect Options: * **B. Decreases:** This would occur with afferent constriction (reduced inflow) or extreme efferent dilatation (reduced back-pressure). * **C. No change:** This is incorrect because the hemodynamics of the nephron are highly sensitive to changes in arteriolar tone. * **D. First increases then decreases:** While *extreme* efferent constriction can eventually decrease GFR (due to a rise in oncotic pressure as plasma flow slows down), the combined effect of afferent dilatation and moderate efferent constriction is a definitive increase. #### NEET-PG High-Yield Pearls: * **ANP (Atrial Natriuretic Peptide):** Naturally causes this exact pattern (afferent dilatation + efferent constriction) to increase GFR and promote sodium excretion. * **Angiotensin II:** At low/physiological doses, it preferentially constricts the **efferent arteriole** to maintain GFR when renal perfusion pressure is low. * **Prostaglandins:** Primarily dilate the **afferent arteriole**. (Note: NSAIDs block this, leading to potential acute kidney injury). * **ACE Inhibitors:** Cause efferent dilatation, which reduces $P_{GC}$—this is why they are renoprotective in diabetes but can cause a drop in GFR in renal artery stenosis.

Question 250: What is true about free water clearance?

A. Regulated by ADH (Correct Answer)
B. Regulated by aldosterone
C. Increased by furosemide
D. None of the above

Explanation: **Explanation:** **Free Water Clearance ($C_{H_2O}$)** represents the volume of blood plasma that is cleared of solute-free water per unit time. It is a measure of the kidney's ability to concentrate or dilute urine. **1. Why Option A is Correct:** The primary regulator of free water clearance is **Antidiuretic Hormone (ADH)**, also known as Vasopressin. ADH acts on the V2 receptors of the principal cells in the collecting ducts to insert **Aquaporin-2** channels. * **High ADH:** Increases water reabsorption, leading to a **negative** free water clearance (concentrated urine). * **Low ADH:** Decreases water reabsorption, leading to a **positive** free water clearance (dilute urine). **2. Why Other Options are Incorrect:** * **Option B:** **Aldosterone** primarily regulates sodium reabsorption and potassium secretion in the distal nephron. While water follows sodium osmotically, aldosterone does not independently regulate "free" (solute-free) water. * **Option C:** **Furosemide** (a loop diuretic) inhibits the Na-K-2Cl cotransporter in the Thick Ascending Limb. This abolishes the corticomedullary osmotic gradient, impairing both the dilution and concentration of urine. Consequently, furosemide brings free water clearance **closer to zero**, rather than increasing it. **3. High-Yield Clinical Pearls for NEET-PG:** * **Formula:** $C_{H_2O} = V - C_{osm}$ (where $V$ is urine flow rate and $C_{osm}$ is osmolar clearance). * **Positive $C_{H_2O}$:** Seen in Diabetes Insipidus and excessive water intake. * **Negative $C_{H_2O}$:** Seen in SIADH and states of dehydration. * **Isosthenuria:** When $C_{H_2O}$ is zero, the urine is iso-osmotic to plasma (often seen in chronic renal failure).

Practice by Chapter

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