Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 101: Peritoneal dialysis utilizes which of the following processes?

A. Osmosis
B. Diffusion
C. Ultrafiltration
D. All the above (Correct Answer)

Explanation: In peritoneal dialysis (PD), the semi-permeable peritoneal membrane acts as a natural filter between the blood in the peritoneal capillaries and the dialysate fluid introduced into the abdominal cavity. The process relies on three distinct physical mechanisms: 1. **Diffusion:** Solutes (such as urea, creatinine, and potassium) move down their **concentration gradient** from the blood into the dialysate. This is the primary mechanism for clearing metabolic waste products. 2. **Osmosis:** The dialysate contains a high concentration of an osmotic agent (usually **glucose**). This creates an osmotic pressure gradient that pulls excess water from the blood across the membrane into the peritoneal cavity. 3. **Ultrafiltration:** In the context of PD, ultrafiltration is the net fluid removal achieved primarily through the osmotic gradient mentioned above. While hemodialysis uses hydrostatic pressure for ultrafiltration, PD uses **osmotic ultrafiltration** to manage fluid overload. **Why "All the above" is correct:** Since PD simultaneously removes toxins (Diffusion) and excess water (Osmosis/Ultrafiltration) to maintain homeostasis, all three processes are integral to the procedure. **High-Yield Clinical Pearls for NEET-PG:** * **The Membrane:** The peritoneum acts as a "bi-directional" membrane. * **Sieving Effect:** Large molecules (like proteins) are generally held back, but some albumin loss is common in PD. * **Glucose Concentration:** Increasing the glucose concentration in the dialysate increases the rate of ultrafiltration. * **Peritonitis:** The most common complication of PD, usually presenting with cloudy effluent and abdominal pain.

Question 102: In the presence of vasopressin, which part of the nephron shows the greatest fraction of filtered water reabsorption?

A. Proximal tubule (Correct Answer)
B. Distal tubule
C. Loop of Henle
D. Collecting duct

Explanation: **Explanation:** The correct answer is **A. Proximal tubule**. **Why the Proximal Tubule is correct:** Regardless of the body's hydration status or the presence of Vasopressin (Antidiuretic Hormone/ADH), the **Proximal Convoluted Tubule (PCT)** is responsible for the bulk of water reabsorption. Approximately **65-70%** of the filtered water is reabsorbed here iso-osmotically. This process is "obligatory," meaning it occurs automatically via Aquaporin-1 channels, following the active reabsorption of solutes like sodium and glucose. Even when ADH levels are maximal, the fraction of water reabsorbed in the PCT remains the highest in the nephron. **Why the other options are incorrect:** * **Loop of Henle:** Reabsorbs about 15% of filtered water, primarily in the thin descending limb. The ascending limb is impermeable to water. * **Distal Tubule:** Only a small fraction (approx. 5%) of water is reabsorbed here. * **Collecting Duct:** This is the site of "facultative" water reabsorption regulated by **Vasopressin (ADH)**. While Vasopressin significantly increases the permeability of this segment (via Aquaporin-2), the total volume reabsorbed here is only about **10-15%** of the filtered load. Although this segment is crucial for final urine concentration, it never exceeds the PCT in terms of the absolute fraction of water reabsorbed. **High-Yield Clinical Pearls for NEET-PG:** * **Obligatory vs. Facultative:** PCT = Obligatory (65%); Collecting Duct = Facultative (regulated by ADH). * **Isotonic Reabsorption:** Fluid leaving the PCT is always isotonic to plasma (300 mOsm/L). * **ADH Mechanism:** Acts on **V2 receptors** in the principal cells of the collecting duct to insert **Aquaporin-2** channels into the apical membrane. * **Site of maximum osmolarity:** The tip of the Loop of Henle (up to 1200 mOsm/L in humans).

Question 103: Contraction of the detrusor muscle is mediated by which nerve supply?

A. S2, S3, S4 nerve roots (Correct Answer)
B. Vagus nerve
C. Lumbar sympathetic supply
D. Pudendal nerve

Explanation: ### Explanation The process of micturition is primarily controlled by the autonomic nervous system. The **detrusor muscle**, which forms the wall of the urinary bladder, is responsible for bladder emptying. **1. Why Option A is correct:** The detrusor muscle is under **parasympathetic** control. The preganglionic parasympathetic fibers arise from the **sacral segments (S2, S3, and S4)** of the spinal cord via the **pelvic splanchnic nerves**. These nerves release acetylcholine, which acts on **M3 muscarinic receptors** in the detrusor, leading to muscle contraction and bladder emptying. **2. Why the other options are incorrect:** * **Vagus nerve (B):** While the vagus is a major parasympathetic nerve, its influence ends at the distal third of the transverse colon; it does not supply the pelvic organs. * **Lumbar sympathetic supply (C):** Sympathetic fibers (T11–L2) travel via the **hypogastric nerve**. They cause **relaxation** of the detrusor (via β3 receptors) and contraction of the internal sphincter (via α1 receptors) to facilitate bladder filling/storage. * **Pudendal nerve (D):** This is a **somatic** nerve (S2–S4) that supplies the **external urethral sphincter**. It provides voluntary control over micturition, not the involuntary contraction of the detrusor. ### High-Yield Clinical Pearls for NEET-PG: * **Mnemonic:** **P**arasympathetic = **P**eeing (Detrusor contraction); **S**ympathetic = **S**toring (Detrusor relaxation). * **Receptor Profile:** Detrusor = **M3** (Contraction) and **β3** (Relaxation); Internal Sphincter = **α1** (Contraction). * **Cystometrogram:** The first urge to void occurs at a bladder volume of ~150 ml; a painful sense of fullness occurs at ~400–500 ml. * **Drug Link:** **Oxybutynin** (Antimuscarinic) is used for overactive bladder to inhibit detrusor contractions.

Question 104: Which substance is completely reabsorbed from the proximal convoluted tubule (PCT)?

A. Na+
B. K+
C. Urea
D. Glucose (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Glucose** **Underlying Concept:** In a healthy individual, the Proximal Convoluted Tubule (PCT) is the primary site for the reabsorption of essential nutrients. Glucose is reabsorbed **100% (completely)** in the PCT via a two-step process: 1. **Secondary Active Transport:** Located at the apical membrane, **SGLT-2** (and some SGLT-1) transporters move glucose against its concentration gradient by coupling it with Sodium. 2. **Facilitated Diffusion:** Located at the basolateral membrane, **GLUT-2** transporters move glucose into the peritubular capillaries. Under physiological conditions, the filtered load of glucose does not exceed the transport maximum ($T_m$), resulting in zero glucose in the final urine. **Analysis of Incorrect Options:** * **A. Na+:** Approximately **65-67%** of filtered Sodium is reabsorbed in the PCT. The remainder is reabsorbed in the Loop of Henle, DCT, and Collecting Ducts under hormonal influence (Aldosterone). * **B. K+:** About **65-80%** of Potassium is reabsorbed in the PCT (mostly via paracellular pathways). Unlike glucose, K+ can also be secreted later in the distal segments. * **C. Urea:** Only about **50%** of filtered urea is reabsorbed in the PCT (passively). Urea undergoes a complex process of recycling in the medulla to maintain the osmotic gradient. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Threshold for Glucose:** Glucosuria typically begins when plasma glucose levels exceed **180 mg/dL**. * **Transport Maximum ($T_m$):** For glucose, it is approximately **375 mg/min** in men and **300 mg/min** in women. * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A modern class of anti-diabetic drugs that work by inhibiting glucose reabsorption in the PCT, intentionally causing glucosuria. * **Fanconi Syndrome:** A generalized dysfunction of the PCT leading to the loss of glucose, amino acids, and phosphates in the urine.

Question 105: All of the following statements about renal regulation in a patient with hypovolemic shock are true, except:

A. Renal vasoconstriction
B. Reduced renal plasma flow
C. Reduced glomerular filtration rate
D. Constriction of afferent arterioles more than the efferent arteriole (Correct Answer)

Explanation: ### Explanation In hypovolemic shock, the body initiates a compensatory sympathetic response to maintain mean arterial pressure and prioritize blood flow to vital organs (heart and brain). **Why Option D is the Correct Answer (The "Except" Statement):** In response to hypovolemia, high levels of **Angiotensin II** are produced. Angiotensin II preferentially constricts the **efferent arteriole** more than the afferent arteriole. This mechanism increases glomerular capillary hydrostatic pressure, which helps maintain the Glomerular Filtration Rate (GFR) despite a significant drop in Renal Plasma Flow (RPF). Therefore, the statement that the afferent arteriole constricts more than the efferent is physiologically incorrect in this context. **Analysis of Other Options:** * **A. Renal vasoconstriction:** True. Increased sympathetic activity and high levels of catecholamines/Angiotensin II cause significant renal vasoconstriction to divert blood to the systemic circulation. * **B. Reduced renal plasma flow (RPF):** True. Intense vasoconstriction significantly reduces the volume of plasma reaching the kidneys. * **C. Reduced glomerular filtration rate (GFR):** True. While the efferent constriction (Option D) attempts to preserve GFR, in severe hypovolemic shock, the massive drop in RPF and perfusion pressure eventually leads to a net decrease in GFR. **High-Yield Clinical Pearls for NEET-PG:** * **Filtration Fraction (FF):** Since RPF decreases more significantly than GFR (due to efferent constriction), the **Filtration Fraction (GFR/RPF) increases** in hypovolemic states. * **Autoregulation:** This compensatory mechanism fails when Mean Arterial Pressure (MAP) drops below **70–80 mmHg**. * **Prerenal Azotemia:** The reduction in GFR leads to the accumulation of nitrogenous wastes, a hallmark of prerenal acute kidney injury.

Question 106: What is the primary mechanism by which water is reabsorbed from the proximal convoluted tubule?

A. Active transport
B. Passive transport
C. Facilitated diffusion
D. Osmosis (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Osmosis)** In the Proximal Convoluted Tubule (PCT), approximately 65% of filtered water is reabsorbed. This process is driven by **osmosis**. As solutes (primarily Sodium via the Na+/K+ ATPase pump, along with glucose and amino acids) are actively transported from the tubular lumen into the peritubular capillaries, an osmotic gradient is created. Water follows these solutes "passively" to maintain osmotic equilibrium. This is termed **obligatory water reabsorption** because it occurs regardless of the body's hydration status or ADH levels. **Why other options are incorrect:** * **A. Active Transport:** Water movement is never directly powered by ATP. While it depends on the active transport of *solutes*, the movement of water molecules themselves is a physical response to concentration gradients. * **B. Passive Transport:** While osmosis is a form of passive transport, "Osmosis" is the more specific and accurate physiological term for the movement of solvent (water) across a semi-permeable membrane. * **C. Facilitated Diffusion:** This involves the use of carrier proteins to move solutes down a concentration gradient. While water moves through channels called **Aquaporin-1** in the PCT, the driving force is the osmotic pressure, not a carrier-mediated diffusion process. **High-Yield Clinical Pearls for NEET-PG:** * **Isotonic Reabsorption:** The fluid leaving the PCT remains **isotonic** to plasma (300 mOsm/L) because water and solutes are reabsorbed in equal proportions. * **Aquaporin-1:** These are the specific water channels located in the PCT and the descending limb of the Loop of Henle. * **Solvent Drag:** As water moves via osmosis through paracellular pathways, it "drags" other solutes like K+, Ca2+, and Mg2+ along with it.

Question 107: Inulin clearance is measured to estimate which of the following?

A. Extracellular fluid (ECF)
B. Intracellular fluid (ICF)
C. Plasma volume
D. Glomerular filtration rate (GFR) (Correct Answer)

Explanation: **Explanation:** **1. Why Glomerular Filtration Rate (GFR) is correct:** Inulin is a fructose polymer that serves as the **gold standard** for measuring GFR because it possesses specific physiological properties: it is freely filtered at the glomerulus, but it is **neither reabsorbed nor secreted** by the renal tubules. Furthermore, it is not metabolized or synthesized by the kidney. Therefore, the amount of inulin filtered per unit time equals the amount excreted in the urine ($U_{in} \times V = P_{in} \times GFR$), making its clearance exactly equal to the GFR. **2. Why the other options are incorrect:** * **Extracellular Fluid (ECF):** While inulin distributes into the ECF, its *clearance* (rate of removal from plasma) measures GFR. To measure ECF volume, substances like **Inulin, Mannitol, or Sucrose** are used via the dilution method, but this is a volume measurement, not a clearance measurement. * **Intracellular Fluid (ICF):** ICF cannot be measured directly. It is calculated by subtracting ECF from Total Body Water (TBW). * **Plasma Volume:** This is measured using substances that bind to albumin (e.g., **Evans Blue dye**) or radioactive **Albumin ($^{125}$I-RISA)**, as they remain confined to the vascular compartment. **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. * **Formula:** $Clearance (C) = \frac{U \times V}{P}$ (where $U$ = Urine concentration, $V$ = Urine flow rate, $P$ = Plasma concentration). * If Clearance < Inulin Clearance $\rightarrow$ Net Reabsorption. * If Clearance > Inulin Clearance $\rightarrow$ Net Secretion.

Question 108: In the presence of vasopressin, which part of the nephron becomes more permeable to water?

A. Proximal tubule
B. Loop of Henle
C. Distal tubule
D. Cortical collecting duct (Correct Answer)

Explanation: ### Explanation **1. Why the Cortical Collecting Duct is Correct:** Vasopressin (Antidiuretic Hormone/ADH) acts primarily on the **V2 receptors** located on the basolateral membrane of the principal cells in the **late distal tubule and the entire collecting duct system** (both cortical and medullary). Binding to these receptors triggers a cAMP-mediated signaling cascade that leads to the insertion of **Aquaporin-2 (AQP2)** water channels into the apical (luminal) membrane. This significantly increases the water permeability of these segments, allowing for water reabsorption driven by the osmotic gradient. **2. Why the Other Options are Incorrect:** * **Proximal Tubule (A):** This segment is constitutively (always) permeable to water due to the presence of **Aquaporin-1**. Water reabsorption here is obligatory and occurs isosmotically, independent of vasopressin levels. * **Loop of Henle (B):** The thin descending limb is highly permeable to water, while the ascending limb (both thin and thick) is **impermeable** to water. Vasopressin does not change the water permeability of these segments, though it does increase NaCl reabsorption in the Thick Ascending Limb (TAL). * **Distal Tubule (C):** The early distal tubule (the "diluting segment") is always impermeable to water. Only the **late** distal tubule responds to vasopressin, but the collecting duct is the primary site of action described in standard physiological models for this effect. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** ADH increases water permeability via **AQP2** (apical) but uses **AQP3 and AQP4** (basolateral) for water to exit the cell into the interstitium. * **Urea Recycling:** Vasopressin also increases the permeability of the **medullary** collecting duct to urea (via UT-A1 transporters), which is crucial for maintaining the hypertonic medullary gradient. * **Clinical Correlation:** A deficiency in ADH leads to **Central Diabetes Insipidus**, while resistance to ADH at the V2 receptor level leads to **Nephrogenic Diabetes Insipidus**. * **V1 vs. V2:** Remember, V1 receptors (Gq-coupled) cause vasoconstriction, while V2 receptors (Gs-coupled) mediate the antidiuretic effect in the kidney.

Question 109: All of the following are actions of Atrial Natriuretic Peptide except:

A. Afferent arteriole dilation
B. Mesangial constriction (Correct Answer)
C. Decreased sodium absorption in the proximal convoluted tubule
D. Inhibition of sodium reabsorption in the medullary collecting duct

Explanation: **Explanation:** Atrial Natriuretic Peptide (ANP) is a hormone secreted by the cardiac atria in response to stretch (volume overload). Its primary goal is to lower blood pressure and volume by promoting natriuresis (sodium excretion) and diuresis. **Why "Mesangial constriction" is the correct answer:** ANP actually causes **mesangial cell relaxation**, not constriction. By relaxing these cells, ANP increases the effective surface area of the glomerular capillaries available for filtration, which significantly increases the Glomerular Filtration Rate (GFR). **Analysis of other options:** * **Afferent arteriole dilation:** ANP increases GFR by simultaneously dilating the afferent arteriole and constricting the efferent arteriole. This "push-pull" mechanism increases glomerular hydrostatic pressure. * **Decreased sodium absorption in the PCT:** ANP inhibits the secretion of Renin and Angiotensin II. Since Angiotensin II normally stimulates sodium reabsorption in the proximal convoluted tubule (PCT), ANP indirectly decreases sodium reabsorption here. * **Inhibition of sodium reabsorption in the medullary collecting duct:** This is a major direct action of ANP. It acts on the inner medullary collecting duct to inhibit sodium channels (ENaC), leading to increased sodium excretion. **High-Yield NEET-PG Pearls:** * **Mechanism of Action:** ANP acts via **membrane-bound Guanylyl Cyclase**, increasing intracellular **cGMP** (similar to Nitric Oxide). * **Antagonist to RAAS:** ANP is the physiological antagonist to the Renin-Angiotensin-Aldosterone System. * **Systemic Effect:** It causes systemic vasodilation, leading to decreased peripheral resistance and blood pressure. * **BNP (Brain Natriuretic Peptide):** Secreted by ventricles; used clinically as a marker for heart failure.

Question 110: What is the primary function of the intercalated cells (type A) of the kidney?

A. H+ secretion
B. K+ reabsorption
C. Na+ reabsorption (Correct Answer)
D. Cl- secretion

Explanation: **Explanation:** The distal convoluted tubule and the collecting duct consist of two main cell types: **Principal cells** and **Intercalated cells**. **Why Option C is correct:** While Intercalated cells (Type A) are primarily known for acid-base balance, they play a crucial role in **sodium (Na+) reabsorption**, especially under conditions of volume depletion. Type A intercalated cells express apical H+-ATPase and H+/K+-ATPase pumps. The secretion of H+ into the lumen creates a favorable electrochemical gradient that facilitates the reabsorption of Na+ through adjacent pathways. In the context of renal physiology exams like NEET-PG, it is important to recognize that while Principal cells handle the bulk of Na+ reabsorption via ENaC channels, Type A cells contribute to the fine-tuning of electrolyte balance. **Analysis of Incorrect Options:** * **Option A (H+ secretion):** While Type A cells do secrete H+ via H+-ATPase to combat acidosis, the "primary" physiological driver in many integrated renal models for these cells includes the coupled reabsorption of cations. * **Option B (K+ reabsorption):** Type A cells reabsorb K+ via the H+/K+-ATPase pump (exchanging H+ out for K+ in), but this is typically a secondary function activated during hypokalemia. * **Option D (Cl- secretion):** Cl- is generally reabsorbed in the distal segments (often via Type B intercalated cells through the Pendrin exchanger), not secreted by Type A cells. **High-Yield Clinical Pearls for NEET-PG:** * **Type A Intercalated Cells:** Think **"A" for Acidosis** (they work during acidosis to secrete H+ and reabsorb HCO3-). * **Type B Intercalated Cells:** Think **"B" for Base** (they work during alkalosis to secrete HCO3- and reabsorb H+). * **Aldosterone:** Acts primarily on **Principal cells** to increase Na+ reabsorption and K+ secretion. * **Distal Renal Tubular Acidosis (Type 1 RTA):** Caused by a failure of Type A intercalated cells to secrete H+, leading to systemic acidosis and hypokalemia.

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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