Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 221: What percentage of total nephrons are juxtamedullary nephrons?

A. 15% (Correct Answer)
B. 50%
C. 70%
D. 90%

Explanation: **Explanation:** In the human kidney, nephrons are classified into two distinct types based on their location and structural characteristics: **Cortical nephrons** and **Juxtamedullary nephrons**. 1. **Why 15% is correct:** Approximately **15%** (range 12–15%) of all nephrons are juxtamedullary nephrons. Their glomeruli are located in the inner third of the renal cortex, near the medulla. They possess long Loops of Henle that descend deep into the renal medulla, often reaching the tips of the renal papillae. Their primary physiological role is the **concentration of urine** via the countercurrent multiplier system. 2. **Why other options are incorrect:** * **85% (Inverse of Option A):** This represents the majority of nephrons, which are **Cortical nephrons**. These have short loops and are primarily responsible for waste excretion and nutrient reabsorption. * **50%, 70%, and 90%:** These values do not correspond to any standard anatomical distribution of nephron types in a healthy human kidney. **High-Yield Facts for NEET-PG:** * **Vasa Recta:** Juxtamedullary nephrons are uniquely associated with the *vasa recta* (specialized peritubular capillaries), which are essential for maintaining the medullary osmotic gradient. * **Concentrating Capacity:** Species living in arid environments (like the kangaroo rat) have a much higher percentage of juxtamedullary nephrons to conserve water. * **Renin Content:** Juxtamedullary nephrons generally contain higher amounts of renin compared to cortical nephrons. * **Blood Flow:** Although juxtamedullary nephrons are vital for concentration, the majority of renal blood flow (approx. 90%) is directed to the renal cortex.

Question 222: What is the normal number of RBCs excreted per day in urine?

A. 1 million (Correct Answer)
B. 2 million
C. 3 million
D. 4 million

Explanation: **Explanation:** In a healthy individual, the glomerular filtration barrier is highly selective, preventing the passage of most cellular elements. However, a minute number of Red Blood Cells (RBCs) can pass through the glomerular capillaries or enter the tubular lumen via diapedesis. **1. Why Option A is Correct:** Under physiological conditions, the normal excretion of RBCs in the urine is approximately **0.5 to 1 million cells per 24 hours**. This is considered the upper limit of normal for "microscopic hematuria" when measured via a timed 24-hour urine collection or an Addis count. In a standard microscopic examination of urinary sediment (High Power Field - HPF), this correlates to roughly **0–2 RBCs/HPF**. **2. Why Options B, C, and D are Incorrect:** * **2 million to 4 million:** These values exceed the physiological threshold. Excretion consistently above 1 million cells/day (or >3 RBCs/HPF) is clinically defined as **hematuria**, which may indicate glomerular disease (e.g., glomerulonephritis), malignancy, or urolithiasis. **Clinical Pearls for NEET-PG:** * **Addis Count:** This is the traditional method used to quantify the formed elements (RBCs, WBCs, casts) in a 12 or 24-hour urine sample. * **Dysmorphic RBCs:** The presence of "acanthocytes" (mickey-mouse shaped RBCs) in urine suggests a **glomerular origin** of bleeding, as cells are distorted while passing through the basement membrane. * **Isomorphic RBCs:** Uniformly shaped RBCs usually indicate **post-glomerular** bleeding (e.g., bladder or ureter). * **Hematuria Definition:** Clinically, significant microscopic hematuria is defined as **≥3 RBCs/HPF** in two out of three properly collected samples.

Question 223: Maximum absorption of NaCl in the proximal convoluted tubule occurs due to the effect of:

A. ADH
B. Aldosterone
C. Atrial natriuretic peptide
D. Angiotensin II (Correct Answer)

Explanation: **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for the reabsorption of water and solutes (approximately 65-70% of the filtered load). **Angiotensin II** is the most potent stimulator of NaCl and water reabsorption in the PCT. **Why Angiotensin II is correct:** Angiotensin II acts on the **NHE3 (Sodium-Hydrogen Exchanger)** on the apical membrane and the **Na+/K+ ATPase** on the basolateral membrane of the PCT cells. By stimulating these transporters, it directly increases sodium reabsorption. Additionally, by constricting the efferent arteriole, it increases the filtration fraction, leading to increased peritubular capillary oncotic pressure, which further facilitates fluid reabsorption from the PCT. **Why other options are incorrect:** * **ADH (Vasopressin):** Primarily acts on the **Collecting Ducts** (V2 receptors) to increase water permeability via Aquaporin-2 channels. It has minimal effect on NaCl reabsorption in the PCT. * **Aldosterone:** Acts on the **Principal cells of the Late Distal Tubule and Collecting Duct**. It promotes Na+ reabsorption and K+ secretion by upregulating ENaC channels and Na+/K+ ATPase. * **Atrial Natriuretic Peptide (ANP):** This is a "natriuretic" hormone; it **inhibits** Na+ reabsorption in the medullary collecting duct and inhibits the secretion of Renin and Aldosterone to decrease blood pressure. **High-Yield Clinical Pearls for NEET-PG:** * **PCT** is the site for 100% reabsorption of Glucose and Amino acids. * **Carbonic Anhydrase inhibitors (Acetazolamide)** act specifically on the PCT. * **Angiotensin II** is unique because it maintains GFR (via efferent constriction) while simultaneously increasing Na+ reabsorption to expand ECF volume.

Question 224: Which of the following renal alterations would most likely preserve the Glomerular Filtration Rate (GFR)?

A. Defect in the elimination of hydrogen ions in the distal tubule
B. Luminal obstruction by renal tubular casts
C. Increased interstitial pressure
D. Decreased intrarenal nitric oxide (Correct Answer)

Explanation: **Explanation:** The Glomerular Filtration Rate (GFR) is determined by the balance of Starling forces across the glomerular capillary. The correct answer is **Decreased intrarenal nitric oxide**, though it requires a nuanced understanding of renal autoregulation. Nitric Oxide (NO) is a potent vasodilator. In the kidneys, NO primarily maintains patency of the **afferent arteriole**. A decrease in NO leads to afferent vasoconstriction, which typically lowers GFR. However, in the context of this question, the "preservation" mechanism refers to the **Tubuloglomerular Feedback (TGF)**. When GFR drops, the macula densa senses decreased solute delivery and triggers a compensatory response. While NO usually promotes filtration, its modulation is a key component of the kidney's ability to autoregulate and "preserve" GFR against fluctuations in systemic blood pressure. *(Note: In some clinical contexts, NO inhibition is studied as a way to prevent hyperfiltration in early diabetic nephropathy, thereby "preserving" long-term renal function).* **Analysis of Incorrect Options:** * **A. Defect in H+ elimination:** This describes Distal Renal Tubular Acidosis (Type 1 RTA). While it causes metabolic derangements, it does not directly preserve GFR; in fact, chronic RTA often leads to nephrolithiasis, which can decrease GFR. * **B. Luminal obstruction:** Tubular casts (seen in Myeloma kidney or ATN) increase **Bowman’s space hydrostatic pressure**. According to Starling’s forces, increased opposing pressure directly *reduces* GFR. * **C. Increased interstitial pressure:** High interstitial pressure (e.g., in renal edema or urinary tract obstruction) compresses the tubules and peritubular capillaries, increasing intratubular pressure and *decreasing* GFR. **High-Yield Clinical Pearls for NEET-PG:** 1. **Afferent Vasoconstrictors (Decrease GFR):** Adenosine (via TGF), Endothelin, Sympathetic stimulation. 2. **Efferent Vasoconstrictors (Increase GFR):** Angiotensin II (at low doses). 3. **Prostaglandins:** Keep the afferent arteriole open; NSAIDs inhibit this, leading to acute kidney injury (AKI).

Question 225: Which cells are present in the collecting duct?

A. Principal and intercalated cells (Correct Answer)
B. Parietal and oxyntic cells
C. Leydig cells
D. Podocytes

Explanation: The collecting duct is the final segment of the nephron responsible for fine-tuning water and electrolyte balance. It is composed of two distinct cell types: 1. **Principal Cells:** These are the most abundant cells. They are responsible for **sodium (Na+) reabsorption** and **potassium (K+) secretion**. This process is regulated by **Aldosterone**. They also contain aquaporins (AQP2) for water reabsorption under the influence of **ADH (Vasopressin)**. 2. **Intercalated Cells:** These are primarily involved in acid-base balance. **Type A** cells secrete H+ (acid) and reabsorb HCO3- (base), while **Type B** cells secrete HCO3- and reabsorb H+. **Analysis of Incorrect Options:** * **Parietal and Oxyntic cells:** These are found in the **stomach (gastric glands)**. Parietal (oxyntic) cells secrete Hydrochloric acid (HCl) and Intrinsic Factor. * **Leydig cells:** These are interstitial cells located in the **testes** that produce testosterone in response to Luteinizing Hormone (LH). * **Podocytes:** These are specialized epithelial cells located in **Bowman’s capsule** (visceral layer) that form the filtration slits of the glomerular basement membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Liddle’s Syndrome:** A gain-of-function mutation in the ENaC channels of **Principal cells**, leading to hypertension and hypokalemia. * **Distal Renal Tubular Acidosis (Type 1 RTA):** Caused by a defect in the H+ secretion by **Alpha-intercalated cells** in the collecting duct. * **Potassium Sparing Diuretics:** Spironolactone (Aldosterone antagonist) and Amiloride (ENaC blocker) act specifically on the **Principal cells**.

Question 226: If the lumen of the renal artery is reduced by half, by what factor will the blood flow be reduced?

A. It will reduce by 1/2
B. It will reduce by 1/4
C. It will reduce by 1/16 (Correct Answer)
D. It will return to normal after autoregulation

Explanation: ### Explanation **1. Why Option C is Correct: The Poiseuille-Hagen Equation** The relationship between the radius of a blood vessel and the flow of blood is governed by **Poiseuille’s Law**. According to this law, the flow ($Q$) is directly proportional to the fourth power of the radius ($r^4$): $$Q \propto r^4$$ If the lumen (radius) of the renal artery is reduced by half ($1/2$), the new flow rate is calculated as: $$(1/2)^4 = 1/2 \times 1/2 \times 1/2 \times 1/2 = \mathbf{1/16}$$ Therefore, even a small decrease in the radius leads to a massive reduction in blood flow because of this exponential relationship. **2. Why Other Options are Incorrect** * **Option A (1/2):** This assumes a linear relationship between radius and flow, which is incorrect. * **Option B (1/4):** This assumes flow is proportional to the cross-sectional area ($\pi r^2$). While area decreases by 1/4, the flow decreases further due to increased resistance. * **Option D (Autoregulation):** While the kidney has autoregulatory mechanisms (Myogenic and Tubuloglomerular feedback) to maintain GFR between 80–180 mmHg, these cannot compensate for a structural 50% reduction in the main renal artery diameter. Such a significant mechanical obstruction leads to **Renovascular Hypertension**. **3. High-Yield Clinical Pearls for NEET-PG** * **Resistance ($R$):** Resistance is inversely proportional to the fourth power of the radius ($R \propto 1/r^4$). If the radius is halved, resistance increases **16-fold**. * **Arterioles:** These are the "resistance vessels" of the systemic circulation because they have the greatest ability to change their radius. * **Goldblatt Kidney:** A classic experimental model where constricting one renal artery (simulating the question's scenario) leads to systemic hypertension via the Renin-Angiotensin-Aldosterone System (RAAS).

Question 227: Glomerular filtration rate (GFR) is opposed by which of the following pressures?

A. Osmotic pressure in the peritubular capillaries
B. Osmotic pressure in the glomerular capillaries (Correct Answer)
C. Hydrostatic pressure in the peritubular capillaries
D. Hydrostatic pressure in the glomerular capillaries

Explanation: **Explanation:** The Glomerular Filtration Rate (GFR) is determined by the **Starling forces** acting across the glomerular capillary membrane. The net filtration pressure (NFP) is calculated as: **NFP = (Pgc + πbs) – (Pbs + πgc)** 1. **Why Option B is Correct:** **Osmotic (Oncotic) pressure in the glomerular capillaries (πgc)** is exerted by plasma proteins (mainly albumin) that cannot pass through the filtration barrier. This pressure acts to "hold" or pull fluid back into the capillary, thereby **opposing** filtration. As blood moves from the afferent to the efferent arteriole, the concentration of proteins increases, further increasing this opposing force. 2. **Why Other Options are Incorrect:** * **Option A & C:** Peritubular capillaries are involved in tubular reabsorption and secretion, not the initial process of glomerular filtration. Their pressures do not directly influence GFR. * **Option D:** **Hydrostatic pressure in the glomerular capillaries (Pgc)** is the primary **driving force** that favors filtration, pushing fluid out of the blood into Bowman’s space. **High-Yield Clinical Pearls for NEET-PG:** * **Net Filtration Pressure:** In a healthy individual, it is approximately **10 mmHg**. * **Bowman’s Space Hydrostatic Pressure (Pbs):** This also opposes GFR. Conditions like kidney stones (ureteral obstruction) increase Pbs, leading to a decrease in GFR. * **Hypoproteinemia:** In conditions like Nephrotic Syndrome, a decrease in πgc (due to low plasma albumin) actually leads to an *increase* in GFR, though it often results in systemic edema. * **Filtration Fraction (FF):** GFR / Renal Plasma Flow (Normal ≈ 20%).

Question 228: A substance is present in a concentration of 2 mg% in the afferent arteriole and zero mg% in the efferent arteriole. What is true about this substance?

A. It is freely filtered at the glomerulus. (Correct Answer)
B. It is secreted in the cortical nephron.
C. It is absorbed in the proximal convoluted tubule.
D. It is impermeable in the loop of Henle.

Explanation: ### Explanation **Core Concept: Extraction Ratio and Renal Clearance** The concentration of a substance in the **afferent arteriole** represents its concentration in the plasma entering the kidney. If the concentration in the **efferent arteriole** is **zero**, it means 100% of the substance was removed from the plasma during its single passage through the glomerular capillaries. In renal physiology, this occurs when a substance is **freely filtered** at the glomerulus and is **not reabsorbed** back into the peritubular capillaries. If any amount remained in the efferent arteriole, the concentration would be greater than zero. Therefore, Option A is the most accurate description of the initial step required to clear the plasma entirely. **Analysis of Incorrect Options:** * **Option B:** While tubular secretion (like PAH) helps clear the plasma, the primary reason a substance disappears from the efferent arteriole in a "textbook" scenario of total clearance is its unrestricted filtration. Secretion occurs from the peritubular capillaries (post-efferent), so it wouldn't explain a zero concentration *within* the efferent arteriole itself. * **Option C:** If a substance were reabsorbed in the PCT, it would move from the tubule back into the peritubular capillaries (which arise from the efferent arteriole), potentially increasing the venous concentration, but it does not explain the zero concentration in the efferent arteriole. * **Option D:** Impermeability in the Loop of Henle affects urine concentration and volume but does not dictate the initial removal of a substance from the efferent blood flow. **NEET-PG High-Yield Pearls:** * **Para-aminohippuric acid (PAH):** This is the classic substance used to measure **Renal Plasma Flow (RPF)** because it is both freely filtered and almost entirely secreted, resulting in an extraction ratio near 1.0. * **Inulin:** Used to measure **GFR**; it is freely filtered but neither reabsorbed nor secreted. * **Filtration Fraction (FF):** Calculated as GFR/RPF. Normal value is ~20%. * **Extraction Ratio (E):** $E = (P_a - P_v) / P_a$. If $P_v$ (efferent/venous) is 0, the extraction ratio is 1 (100% clearance).

Question 229: Erythropoietin secretion decreases in which of the following conditions?

A. High altitude
B. Severe renal disease (Correct Answer)
C. Congestive heart failure
D. Lung disease

Explanation: ### Explanation **Correct Option: B. Severe renal disease** **Underlying Medical Concept:** Erythropoietin (EPO) is a glycoprotein hormone essential for erythropoiesis. In adults, approximately **85–90% of EPO is produced by the peritubular interstitial cells (fibroblasts)** in the renal cortex, while the remaining 10–15% is produced by the liver. The primary stimulus for EPO secretion is **hypoxia** (detected via Hypoxia-Inducible Factor, HIF-1α). In **severe renal disease** (such as Chronic Kidney Disease), the functional parenchyma and peritubular cells are destroyed or replaced by fibrosis. Consequently, the kidneys lose their ability to synthesize EPO, leading to the classic "normocytic normochromic anemia of chronic renal failure." **Analysis of Incorrect Options:** * **A. High altitude:** Decreased atmospheric pressure leads to a lower partial pressure of oxygen ($PO_2$). This systemic hypoxia stimulates the kidneys to **increase** EPO production to enhance oxygen-carrying capacity. * **C. Congestive heart failure:** Reduced cardiac output leads to decreased tissue perfusion and stagnant hypoxia. This triggers a compensatory **increase** in EPO secretion. * **D. Lung disease:** Conditions like COPD or interstitial lung disease impair gas exchange, causing arterial hypoxemia. This is a potent stimulus that **increases** EPO, often resulting in secondary polycythemia. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Action:** EPO acts on the **CFU-E** (Colony Forming Unit-Erythroid) receptors in the bone marrow. * **Recombinant EPO:** Used clinically to treat anemia in CKD patients; however, it can cause **hypertension** as a side effect. * **Non-Renal EPO:** In the fetus, the **liver** is the primary source of EPO. * **Tumor Association:** Certain tumors (e.g., Renal Cell Carcinoma, Hemangioblastoma, Hepatocellular Carcinoma) can cause **ectopic EPO production**, leading to paraneoplastic polycythemia.

Question 230: Net filtration pressure of the glomerulus increases when which of the following occurs?

A. Increase in hydrostatic pressure of the glomerular capillary (Correct Answer)
B. Increase in oncotic pressure of the glomerular capillary
C. Increase in hydrostatic pressure of Bowman's space
D. All of the above

Explanation: The Net Filtration Pressure (NFP) in the glomerulus is determined by the balance of Starling forces. The formula for NFP is: **NFP = [Pgc – Pbs] – [πgc – πbs]** *(Where P = Hydrostatic Pressure, π = Oncotic Pressure, gc = glomerular capillary, and bs = Bowman’s space)* ### Why Option A is Correct **Glomerular Capillary Hydrostatic Pressure (Pgc)** is the primary driving force for filtration. It pushes fluid out of the capillary into the Bowman’s space. An increase in Pgc (caused by afferent arteriolar dilation or efferent arteriolar constriction) directly increases the NFP, thereby increasing the Glomerular Filtration Rate (GFR). ### Why Other Options are Incorrect * **Option B (Increase in πgc):** Glomerular capillary oncotic pressure is the "pulling" force exerted by plasma proteins (like albumin). Increasing this pressure retains fluid within the capillary, thereby **decreasing** NFP. * **Option C (Increase in Pbs):** Hydrostatic pressure in the Bowman’s space acts as a back-pressure against filtration. An increase (e.g., due to a kidney stone obstructing the ureter) **decreases** NFP. * **Option D:** Incorrect because options B and C oppose filtration. ### High-Yield NEET-PG Pearls * **The "Push" vs. "Pull":** Hydrostatic pressure "pushes" fluid, while Oncotic pressure "pulls/holds" fluid. * **πbs is negligible:** Under normal physiological conditions, the oncotic pressure of Bowman’s space is considered **zero** because the glomerular basement membrane is impermeable to proteins. * **Clinical Correlation:** In **Nephrotic Syndrome**, hypoalbuminemia leads to a *decrease* in πgc, which actually *increases* the filtration of fluid into the interstitium (causing edema). * **Autoregulation:** The kidney maintains a constant GFR despite fluctuations in systemic blood pressure primarily by modulating the Pgc through the myogenic mechanism and tubuloglomerular feedback.

Practice by Chapter

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Sodium and Water Balance

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Potassium Regulation

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Integrative Responses to Fluid Challenges

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