Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 281: What is the most important buffer system present in the distal convoluted tubule?

A. Bicarbonate
B. Phosphate
C. Protein
D. Ammonia (Correct Answer)

Explanation: **Explanation:** The primary role of the distal nephron in acid-base balance is the excretion of fixed acids ($H^+$ ions). While the bicarbonate buffer system is crucial in the extracellular fluid and proximal tubule, it is not the dominant buffer in the distal tubule because most bicarbonate has already been reabsorbed. **Why Ammonia is the Correct Answer:** The **Ammonia ($NH_3/NH_4^+$) buffer system** is the most important and physiologically flexible buffer in the distal tubule. Unlike other buffers, its synthesis can be **upregulated** in response to chronic acidosis. Ammonia is produced in the proximal tubule from Glutamine, but it exerts its major buffering effect in the collecting ducts. $NH_3$ diffuses into the tubular lumen, combines with secreted $H^+$ to form $NH_4^+$ (ammonium), which is "trapped" and excreted. This system accounts for about **60-75%** of net acid excretion. **Analysis of Incorrect Options:** * **Bicarbonate:** While it is the most important ECF buffer, 80-90% is reabsorbed in the proximal tubule. By the time fluid reaches the distal tubule, its concentration is too low to be the primary buffer. * **Phosphate:** Known as the "Titratable Acid" buffer. Although important, its concentration is fixed and cannot be increased during acidosis, making it less significant than the ammonia system. * **Protein:** Proteins are major intracellular and plasma buffers but are normally absent (or present in negligible amounts) in the tubular fluid of a healthy nephron. **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The production of ammonia from **Glutamine** by the enzyme **Glutaminase** is the key adaptive response to metabolic acidosis. * **Diffusion Trapping:** $NH_3$ is lipid-soluble and diffuses easily; $NH_4^+$ is water-soluble and trapped in the lumen (cannot diffuse back). * **Net Acid Excretion (NAE):** Calculated as $[NH_4^+ \text{ excretion} + \text{Titratable Acid}] - [\text{Bicarbonate excretion}]$.

Question 282: What percentage of glucose is absorbed in the proximal convoluted tubule?

A. 100% (Correct Answer)
B. 0%
C. 60%
D. 50%

Explanation: **Explanation:** In a healthy individual, **100% of filtered glucose is reabsorbed in the Proximal Convoluted Tubule (PCT)**. Glucose is a vital nutrient, and the kidneys are designed to ensure none is lost in the urine under normal physiological conditions. **Mechanism:** Reabsorption occurs via **Secondary Active Transport**. * **SGLT-2 (Sodium-Glucose Co-transporter 2):** Located in the early PCT (S1 segment), it reabsorbs about 90% of glucose. * **SGLT-1:** Located in the late PCT (S3 segment), it reabsorbs the remaining 10%. * Once inside the tubular cell, glucose exits into the blood via facilitated diffusion through **GLUT-2** (early PCT) and **GLUT-1** (late PCT). **Analysis of Incorrect Options:** * **B (0%):** This would imply total glycosuria, seen only in severe pathological states or specific drug actions. * **C & D (60% & 50%):** While the PCT reabsorbs roughly 65% of water and electrolytes (like Na+ and K+), it is specialized to reabsorb 100% of glucose and amino acids. **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Threshold for Glucose:** Glucose starts appearing in urine (glycosuria) when blood glucose levels exceed **180 mg/dL**. 2. **Transport Maximum ($T_m$):** The point where all glucose transporters are saturated. For men, $T_mG$ is approx. **375 mg/min**; for women, **300 mg/min**. 3. **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A modern class of anti-diabetic drugs that work by inhibiting glucose reabsorption in the PCT, purposely inducing glycosuria to lower blood sugar. 4. **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, and phosphates in the urine.

Question 283: What is true about the macula densa?

A. It is also known as tubuloglomerular feedback.
B. The macula densa senses changes in NaCl concentration.
C. It is involved in the autoregulation of GFR.
D. All of the above. (Correct Answer)

Explanation: The **macula densa** is a specialized cluster of tall, closely packed epithelial cells located in the initial segment of the distal convoluted tubule, where it comes into contact with the afferent and efferent arterioles of its parent nephron. ### **Explanation of Options:** * **Option B (NaCl Sensing):** The primary function of the macula densa is to act as a **chemoreceptor**. It senses changes in the delivery of **Sodium Chloride (NaCl)** to the distal tubule via the NKCC2 transporter. * **Option A & C (TGF and Autoregulation):** When the macula densa detects an increase in NaCl (indicating high GFR), it triggers the release of adenosine, which causes vasoconstriction of the afferent arteriole. This mechanism is known as **Tubuloglomerular Feedback (TGF)**. TGF is one of the two primary mechanisms (alongside the myogenic reflex) responsible for the **Autoregulation of GFR**, ensuring it remains stable despite fluctuations in systemic blood pressure. Since all statements accurately describe the function and context of the macula densa, **Option D** is the correct answer. ### **High-Yield Clinical Pearls for NEET-PG:** * **Juxtaglomerular Apparatus (JGA):** Comprises the Macula Densa, Juxtaglomerular (JG) cells (modified smooth muscle of afferent arteriole), and Lacis cells (extraglomerular mesangial cells). * **Renin Release:** When NaCl levels are **low** (indicating low BP/GFR), the macula densa signals the JG cells to release **Renin**, activating the RAAS pathway. * **Transporter:** The macula densa senses NaCl specifically through the **NKCC2** (Sodium-Potassium-2 Chloride) symporter. * **Signaling Molecule:** Adenosine is the primary mediator for vasoconstriction in TGF, while Prostaglandins (PGE2) and Nitric Oxide (NO) are involved when vasodilation is required.

Question 284: A luminal cell membrane Na+ channel is the main pathway for Na+ reabsorption in which of the following cell types?

A. Proximal tubule cells
B. Thick ascending limb cells
C. Distal convoluted tubule cells
D. Collecting duct principal cells (Correct Answer)

Explanation: **Explanation:** The correct answer is **Collecting duct principal cells**. The question asks for the cell type where Na+ reabsorption occurs via a **luminal Na+ channel** (rather than a symporter or antiporter). 1. **Why Principal Cells are correct:** In the late distal tubule and cortical collecting duct, Na+ reabsorption is mediated by the **Epithelial Sodium Channel (ENaC)** located on the apical (luminal) membrane. This is a selective channel, not a transporter. This process is electrogenic and is the primary site of action for **Aldosterone**, which increases the number and activity of ENaCs. 2. **Why other options are incorrect:** * **Proximal Tubule Cells:** Na+ reabsorption here occurs primarily via the **Na+-H+ exchanger (NHE3)** and various symporters (e.g., Na+-Glucose, Na+-Amino Acid). * **Thick Ascending Limb (TAL) Cells:** Na+ enters these cells via the **NKCC2 symporter** (Na+-K+-2Cl- cotransporter), which is the target of loop diuretics. * **Distal Convoluted Tubule (DCT) Cells:** Na+ reabsorption is mediated by the **NCC symporter** (Na+-Cl- cotransporter), which is inhibited by thiazide diuretics. **High-Yield Clinical Pearls for NEET-PG:** * **Liddle Syndrome:** A genetic mutation causing "gain of function" in ENaC, leading to hypertension, hypokalemia, and metabolic alkalosis (mimics hyperaldosteronism but with low renin/aldosterone). * **Amiloride & Triamterene:** These are potassium-sparing diuretics that specifically block the **ENaC** in the principal cells. * **ANP (Atrial Natriuretic Peptide):** Inhibits ENaC, promoting natriuresis.

Question 285: Which of the following statements is FALSE regarding type 2 renal tubular acidosis?

A. Proximal tubule defect
B. Urine pH > 5.5 (Correct Answer)
C. Normal anion gap
D. Fanconi syndrome present

Explanation: ### Explanation **Type 2 Renal Tubular Acidosis (RTA)**, also known as **Proximal RTA**, is characterized by a defect in the proximal convoluted tubule’s (PCT) ability to reabsorb filtered bicarbonate ($HCO_3^-$). #### Why Option B is the Correct (False) Statement: In Type 2 RTA, although the proximal tubule fails to reabsorb bicarbonate initially, the distal tubule remains functional. Once the plasma bicarbonate level drops below the "renal threshold" (typically 12–20 mEq/L), the distal nephron is capable of acidifying the urine normally. Therefore, in a steady state of chronic acidemia, the **urine pH is typically < 5.5**. A urine pH consistently > 5.5 is a hallmark of **Type 1 (Distal) RTA**, where the distal tubule cannot secrete $H^+$ ions. #### Analysis of Incorrect Options: * **Option A (Proximal tubule defect):** This is true. The primary pathology is the loss of carbonic anhydrase activity or sodium-bicarbonate cotransporters in the PCT. * **Option C (Normal anion gap):** This is true. All RTAs are characterized by a **Normal Anion Gap Metabolic Acidosis (NAGMA)**, as the loss of $HCO_3^-$ is compensated by an increase in serum Chloride ($Cl^-$), leading to hyperchloremia. * **Option D (Fanconi syndrome present):** This is true. Type 2 RTA is frequently associated with generalized proximal tubule dysfunction (Fanconi Syndrome), leading to phosphaturia, glycosuria, aminoaciduria, and uricosuria. #### High-Yield Clinical Pearls for NEET-PG: * **Type 1 RTA (Distal):** Inability to secrete $H^+$; Urine pH > 5.5; associated with hypokalemia and **nephrolithiasis** (calcium phosphate stones). * **Type 2 RTA (Proximal):** Inability to reabsorb $HCO_3^-$; Urine pH < 5.5 (eventually); associated with **osteomalacia/rickets**. * **Type 4 RTA (Hyperkalemic):** Associated with **Aldosterone deficiency or resistance**; the only RTA with **high serum potassium**. * **Mnemonic:** "Bi-**P**-ar-**T**-ite" — **P**roximal is Type **2**.

Question 286: Glomerular Filtration Rate is increased when?

A. Plasma oncotic pressure is increased
B. Glomerular hydrostatic pressure is decreased
C. Tubular hydrostatic pressure is decreased
D. Increased renal blood flow (Correct Answer)

Explanation: **Explanation:** The Glomerular Filtration Rate (GFR) is governed by **Starling’s Forces**, expressed by the formula: **GFR = Kf × [(Pgc – Pbs) – (πgc – πbs)]** *(Where P = Hydrostatic pressure, π = Oncotic pressure, gc = glomerular capillary, and bs = Bowman’s space).* **Why Option D is Correct:** **Increased Renal Blood Flow (RBF)** is a primary driver of GFR. When RBF increases, the delivery of plasma to the glomerulus increases. This maintains a high hydrostatic pressure along the length of the capillary and prevents the rapid rise of oncotic pressure (due to filtration), thereby increasing the net filtration pressure. **Analysis of Incorrect Options:** * **A. Plasma oncotic pressure is increased:** Increased πgc (e.g., in dehydration or multiple myeloma) exerts a "suction" force that retains fluid in the capillary, thereby **decreasing** GFR. * **B. Glomerular hydrostatic pressure is decreased:** Pgc is the primary force favoring filtration. A decrease (due to hypotension or afferent arteriole constriction) leads to a **decrease** in GFR. * **C. Tubular hydrostatic pressure is decreased:** Actually, a decrease in Pbs (Bowman’s space pressure) would **increase** GFR because there is less resistance to fluid entering the tubule. However, in clinical scenarios like urinary tract obstruction (stones), Pbs increases, which decreases GFR. **High-Yield Clinical Pearls for NEET-PG:** * **Afferent Arteriole:** Constriction decreases GFR; Dilatation (via Prostaglandins) increases GFR. * **Efferent Arteriole:** Moderate constriction **increases** GFR (by increasing Pgc); however, severe constriction may decrease GFR due to a drastic drop in RBF. * **Creatinine Clearance:** The most common clinical marker used to estimate GFR. * **Inulin Clearance:** The "Gold Standard" for measuring GFR because it is freely filtered but neither reabsorbed nor secreted.

Question 287: The juxtaglomerular apparatus is located in relation to which structure?

A. Proximal convoluted tubule
B. Ascending loop of Henle
C. Descending loop of Henle
D. Glomerulus (Correct Answer)

Explanation: **Explanation:** The **Juxtaglomerular Apparatus (JGA)** is a specialized structure formed by the distal part of the thick ascending limb of the loop of Henle (specifically the **Macula Densa**) coming into direct contact with the afferent and efferent arterioles of its parent nephron. This contact occurs at the **vascular pole of the Glomerulus**. Its primary function is to regulate blood pressure and the glomerular filtration rate (GFR) through the Renin-Angiotensin-Aldosterone System (RAAS) and tubuloglomerular feedback. **Why the other options are incorrect:** * **Proximal Convoluted Tubule (PCT):** The PCT is located at the urinary pole of the glomerulus, opposite to the JGA. It is primarily involved in bulk reabsorption, not autoregulation. * **Ascending/Descending Loop of Henle:** While the Macula Densa is technically the terminal part of the thick ascending limb, the JGA as a functional unit (comprising JG cells, Lacis cells, and Macula Densa) is defined by its anatomical relationship and physical proximity to the **Glomerulus** and its arterioles. **High-Yield NEET-PG Pearls:** 1. **Components of JGA:** * **Juxtaglomerular (JG) cells:** Modified smooth muscle cells in the *afferent arteriole* that act as baroreceptors and secrete **Renin**. * **Macula Densa:** Specialized cells in the distal tubule that act as **chemoreceptors** (sensing NaCl levels). * **Lacis Cells (Extraglomerular Mesangial cells):** Provide structural support and signaling. 2. **Function:** It is the site of **Tubuloglomerular Feedback (TGF)**—if NaCl at the macula densa increases, the JGA causes afferent arteriolar constriction to decrease GFR. 3. **Innervation:** JG cells are primarily innervated by **Sympathetic (Beta-1)** receptors, which stimulate renin release.

Question 288: Which of the following statements regarding bladder function is true?

A. Bladder emptying cannot occur if the volume is less than 100 ml. (Correct Answer)
B. Bladder muscles contain intrafusal fibres.
C. Pressure in the bladder increases very linearly with time.
D. A single instance of fluid intake results in complete excretion in 1 hour.

Explanation: **Explanation:** **Correct Option: A** The micturition reflex is a stretch-regulated autonomic reflex. In adults, the first urge to void is typically felt at a bladder volume of **150 ml**, and a marked sense of fullness occurs at **300–400 ml**. While the bladder can technically contract at lower volumes, functional bladder emptying (micturition) generally does not occur if the volume is less than **100 ml** because the stretch receptors in the bladder wall are not sufficiently stimulated to trigger the parasympathetic reflex arc required for detrusor contraction and sphincter relaxation. **Incorrect Options:** * **B:** Bladder muscles (the detrusor) are **smooth muscles** and do not contain intrafusal fibers. Intrafusal fibers are specialized sensory organs found only in **skeletal muscles** (muscle spindles) to detect changes in length. * **C:** The bladder exhibits **plasticity** and **compliance**. Due to the Law of Laplace, as the bladder fills, the pressure remains relatively constant (plateau phase) despite increasing volume. It is not a linear increase; pressure only rises sharply once the bladder reaches its functional capacity. * **D:** Renal excretion of a water load is not instantaneous. It typically takes about **2–3 hours** for a single large fluid intake to be completely excreted, as the process depends on GFR, ADH suppression, and tubular processing. **High-Yield Clinical Pearls for NEET-PG:** * **Cystometrogram:** The graphical representation of intravesical pressure vs. volume. * **Nerve Supply:** Parasympathetic (S2-S4 via Pelvic nerve) causes contraction; Sympathetic (L1-L2 via Hypogastric nerve) causes filling/relaxation. * **Internal Sphincter:** Involuntary (Smooth muscle); **External Sphincter:** Voluntary (Skeletal muscle - Pudendal nerve).

Question 289: What is the typical blood flow rate through the kidneys?

A. 1-1.5 L/min (Correct Answer)
B. 1.5-2 L/min
C. 2-2.5 L/min
D. 2.5-3 L/min

Explanation: **Explanation:** The correct answer is **A (1-1.5 L/min)**. In a healthy adult weighing approximately 70 kg, the Renal Blood Flow (RBF) is roughly **1100 to 1200 mL/min**. This represents about **20-25% of the total cardiac output** (CO), despite the kidneys accounting for less than 0.5% of total body weight. This high flow rate is not required for the metabolic demands of the renal tissue itself, but rather to ensure a high Glomerular Filtration Rate (GFR) for effective regulation of body fluids and waste excretion. **Analysis of Options:** * **Option A (1-1.5 L/min):** This range accurately encompasses the physiological norm of 1.1–1.2 L/min. * **Options B, C, and D:** These values (1.5 L/min to 3 L/min) significantly overestimate the RBF. A blood flow of 2.5-3 L/min would represent nearly 50-60% of the cardiac output, which is physiologically impossible under normal resting conditions. **High-Yield Facts for NEET-PG:** * **Renal Plasma Flow (RPF):** Since hematocrit is ~45%, the plasma flow is roughly 55% of RBF, equating to approximately **600-650 mL/min**. * **Filtration Fraction (FF):** Calculated as GFR/RPF. Normal value is ~0.2 (meaning 20% of plasma entering the kidney is filtered). * **Oxygen Consumption:** The kidneys have the highest oxygen consumption per gram of tissue after the heart, primarily used for active sodium reabsorption in the tubules. * **Autoregulation:** RBF and GFR remain constant over a wide range of mean arterial pressures (**80–180 mmHg**) due to myogenic mechanisms and tubuloglomerular feedback.

Question 290: What is the primary function of Antidiuretic Hormone (ADH)?

A. Promotes water reabsorption (Correct Answer)
B. Inhibits water excretion
C. Facilitates sodium absorption
D. Stimulates potassium secretion

Explanation: **Explanation:** **Antidiuretic Hormone (ADH)**, also known as Vasopressin, is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and secreted by the posterior pituitary. Its primary role is the regulation of plasma osmolality. **Why Option A is Correct:** ADH acts on the **V2 receptors** located on the basolateral membrane of the **principal cells** in the late distal tubule and collecting ducts. This triggers a cAMP-mediated signaling pathway that leads to the insertion of **Aquaporin-2 (AQP2)** water channels into the apical membrane. This increases the permeability of the collecting duct to water, allowing water to be reabsorbed down the osmotic gradient into the hypertonic renal medullary interstitium. **Why Other Options are Incorrect:** * **Option B:** While ADH results in decreased water excretion, this is a *consequence* of its primary physiological action, which is active reabsorption. * **Option C:** Sodium reabsorption is primarily regulated by **Aldosterone** (acting on ENaC channels). While ADH can have minor effects on sodium transporters (NKCC2), it is not its primary function. * **Option D:** Potassium secretion is a function of Aldosterone and tubular flow rates, not ADH. **High-Yield Clinical Pearls for NEET-PG:** * **V1 Receptors:** Located on vascular smooth muscle; cause vasoconstriction (hence the name Vasopressin). * **Diabetes Insipidus (DI):** Deficiency of ADH (Central DI) or resistance to ADH (Nephrogenic DI) leads to polyuria and dilute urine. * **SIADH:** Excessive ADH secretion leads to water retention, dilutional hyponatremia, and concentrated urine. * **Urea Recycling:** ADH also increases the permeability of the medullary collecting duct to urea (via UT-A1 transporters), which helps maintain the medullary osmotic gradient.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

Tubular Reabsorption and Secretion

Practice Questions

Concentration and Dilution of Urine

Practice Questions

Acid-Base Regulation by the Kidneys

Practice Questions

Sodium and Water Balance

Practice Questions

Potassium Regulation

Practice Questions

Calcium and Phosphate Handling

Practice Questions

Micturition Physiology

Practice Questions

Renal Function Tests

Practice Questions

Integrative Responses to Fluid Challenges

Practice Questions

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