Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 511: Mechanism of secretion of ammonia in distal tubule is?

A. Primary active transport
B. Symport
C. Antiport
D. Passive diffusion (Correct Answer)

Explanation: ***Passive diffusion*** - Ammonia (NH3) is a **lipid-soluble molecule** that can readily cross cell membranes, including those of the distal tubule and collecting duct, down its **concentration gradient**. - This process is crucial for regulating **acid-base balance**, as NH3 traps H+ ions to form NH4+, which is then excreted. *Primary active transport* - This mechanism involves the direct use of **ATP hydrolysis** to move ions against their concentration gradient, which is not the primary way ammonia is secreted in the distal tubule. - While NH4+ can be secreted via active transport in some segments (e.g., substituting for K+ on the Na-K-2Cl cotransporter in the thick ascending limb), free ammonia diffusion is distinct. *Symport* - **Symport** involves the co-transport of two or more different molecules or ions in the same direction across a cell membrane, powered by an electrochemical gradient. - This mechanism is not typically involved in the secretion of uncharged, lipid-soluble ammonia. *Antiport* - **Antiport** is a type of coupled transport where two different ions or molecules move in opposite directions across a membrane. - While antiport systems are essential for various renal functions (e.g., Na+/H+ exchanger), they are not the primary mechanism for the secretion of free ammonia in the distal tubule.

Question 512: Which of the following is the primary factor involved in mesangial cell contraction?

A. ANP
B. Endothelin-1
C. Angiotensin II (Correct Answer)
D. Platelet-activating factor (PAF)

Explanation: ***Angiotensin II*** - **Angiotensin II** is a potent vasoconstrictor that directly stimulates **mesangial cell contraction**. - Contraction of mesangial cells reduces the **glomerular surface area** available for filtration, thereby decreasing the **glomerular filtration rate (GFR)**. *Endothelin-1* - **Endothelin-1** is a potent vasoconstrictor produced by endothelial cells, which can also induce mesangial cell contraction. - However, its role in **mesangial cell contraction** is generally considered secondary to **angiotensin II** in physiological regulation. *ANP* - **Atrial natriuretic peptide (ANP)** is a hormone that causes **vasodilation** and relaxation of mesangial cells. - Its primary effect is to **increase GFR** and sodium excretion, opposing the effects of vasoconstrictors. *Platelet-activating factor (PAF)* - PAF is a **phospholipid mediator** involved in inflammation and allergic reactions. - While it can affect renal hemodynamics, its role in directly and primarily causing **mesangial cell contraction** is less significant compared to angiotensin II.

Question 513: Increased aldosterone and ADH secretion following major trauma results in all the following except?

A. Increased osmolarity of urine
B. Increased water excretion (Correct Answer)
C. Increased K+ excretion in urine
D. Decreased Na+ excretion in urine

Explanation: ***Increased water excretion*** - **ADH (antidiuretic hormone)** increases water reabsorption in the collecting ducts, leading to a *decrease* in water excretion, not an increase. - Increased aldosterone and ADH would promote fluid retention to maintain blood volume following trauma, thus reducing water loss via urine. *Decreased Na+ excretion in urine* - **Aldosterone** acts on the renal tubules to increase **sodium reabsorption** and potassium excretion. - This response is crucial in **conserving sodium** and thereby maintaining extracellular fluid volume after trauma. *Increased K+ excretion in urine* - **Aldosterone** directly stimulates **potassium secretion** into the urine in the principal cells of the collecting ducts. - This is a normal physiological consequence of increased aldosterone levels. *Increased osmolarity of urine* - **ADH** increases the permeability of the collecting ducts to water, leading to **more water reabsorption** back into the bloodstream. - This removal of water from the urine concentrates the solutes, resulting in a **more concentrated (higher osmolarity)** urine.

Question 514: Which of the following is the MOST important factor determining whether a substance can be filtered at the glomerulus?

A. Lipid solubility of the substance
B. Molecular weight of the substance (Correct Answer)
C. Binding capacity to albumin
D. None of the options

Explanation: ***Molecular weight of the substance*** - The **glomerular filtration barrier** acts as a size-selective filter, generally permeable to substances with a molecular weight less than 5,000-10,000 Daltons - Larger molecules are typically restricted from filtration due to the **size exclusion** property of the glomerular basement membrane and podocyte slit diaphragms - This is the **primary determinant** of whether a substance can be filtered at all, making it the most important factor among the given options *Lipid solubility of the substance* - **Lipid solubility** is more relevant for reabsorption and secretion in the renal tubules, particularly for passive diffusion across tubular cell membranes - It has minimal direct influence on the initial filtration process at the glomerulus, which is primarily a **pressure-driven, size- and charge-selective ultrafiltration** process - The glomerular capillary wall is not a lipid membrane barrier for the filtration process *Binding capacity to albumin* - Substances bound to **large plasma proteins** like albumin (molecular weight ~67,000 Daltons) cannot pass through the glomerular filtration barrier - While important for determining the *free, filterable fraction* of a substance in plasma, the binding itself is secondary to the fundamental molecular weight/size restriction - Only the **free (unbound) fraction** of a substance is available for filtration, and whether it filters depends primarily on its molecular weight *None of the options* - This option is incorrect because **molecular weight** is indeed the most critical factor among the given options for determining whether a substance can be filtered at the glomerulus

Question 515: A substance has a clearance similar to inulin clearance. How is this substance primarily excreted in urine?

A. Tubular Secretion
B. Glomerular filtration (Correct Answer)
C. Vascular leakage
D. Both tubular secretion and glomerular filtration

Explanation: ***Glomerular filtration*** - **Inulin** is a gold standard for measuring **glomerular filtration rate** (GFR) because it is freely filtered by the glomeruli and is neither reabsorbed nor secreted by the renal tubules. - Therefore, a substance with clearance similar to inulin is primarily excreted via **glomerular filtration**. *Tubular Secretion* - If a substance were primarily excreted by tubular secretion, its clearance would be **higher than the GFR**, as secretion adds more of the substance to the urine than filtration alone. - This mechanism is characteristic of substances like **para-aminohippurate (PAH)**, which is used to measure renal plasma flow. *Vascular leakage* - **Vascular leakage** is not a normal mechanism of substance excretion in the urine. - It refers to the abnormal passage of fluid and macromolecules from blood vessels into tissues, often seen in conditions like inflammation or sepsis, and does not directly contribute to renal clearance. *Both tubular secretion and glomerular filtration* - If a substance were excreted by both **tubular secretion and glomerular filtration**, its clearance would also be **higher than the GFR**, similar to substances that undergo significant tubular secretion. - The fact that its clearance is *similar* to inulin specifically points to filtration as the predominant and almost exclusive mechanism.

Question 516: Tubuloglomerular feedback is for regulation of?

A. BP
B. Blood volume
C. Na+ reabsorption
D. Renal blood flow (Correct Answer)

Explanation: ***Renal blood flow*** - **Tubuloglomerular feedback (TGF)** is a key mechanism that helps to tightly regulate **renal blood flow** and **glomerular filtration rate (GFR)** by sensing tubular fluid composition. - This mechanism involves the **macula densa** cells in the distal tubule sensing changes in **sodium chloride (NaCl) delivery**, which then signals the afferent arteriole to adjust its caliber. *BP* - While renal blood flow ultimately influences **blood pressure (BP)**, tubuloglomerular feedback is primarily focused on **local autoregulation** within the kidney, not systemic BP control. - Systemic BP is regulated by much broader mechanisms involving the **renin-angiotensin-aldosterone system** and **autonomic nervous system**. *Blood volume* - **Blood volume** is primarily regulated by hormones like **ADH (vasopressin)** and **aldosterone**, which influence water and sodium reabsorption in the collecting ducts and other parts of the nephron. - Although renal function impacts blood volume, tubuloglomerular feedback's direct role is not in regulating the overall volume of blood. *Na+ reabsorption* - While the macula densa senses **Na+ delivery** to the distal tubule and this influences GFR, the primary role of tubuloglomerular feedback is not to modulate the total amount of **Na+ reabsorbed** throughout the nephron. - Na+ reabsorption is regulated by multiple factors and occurs extensively in the **proximal tubule**, **loop of Henle**, and **distal tubule**, often under hormonal control.

Question 517: What is the primary function of tubuloglomerular feedback?

A. Regulation of blood pressure
B. Regulation of blood volume
C. Regulation of sodium reabsorption
D. Regulation of glomerular filtration rate (Correct Answer)

Explanation: ***Regulation of glomerular filtration rate*** - **Tubuloglomerular feedback (TGF)** is a key intrinsic mechanism that regulates **glomerular filtration rate (GFR)** by sensing changes in the tubular fluid composition at the macula densa. - It involves signaling between the **macula densa** cells of the distal tubule and the afferent arteriole, adjusting the arterial tone to maintain a stable GFR. *Regulation of blood pressure* - While GFR regulation can indirectly affect blood pressure, the primary and direct function of TGF is not **blood pressure regulation**. - Blood pressure is primarily controlled by systemic mechanisms involving the **renin-angiotensin-aldosterone system** and autonomic nervous system. *Regulation of blood volume* - **Blood volume** is regulated by various hormonal and neural mechanisms affecting **sodium and water reabsorption**, such as ADH and aldosterone. - TGF influences fluid filtration, which can affect overall fluid balance, but its direct role is not the primary regulation of blood volume. *Regulation of sodium reabsorption* - TGF senses **sodium concentration** in the tubular fluid at the macula densa, but its primary effect is on the GFR, not directly on the regulation of **sodium reabsorption** in other parts of the nephron. - Sodium reabsorption is primarily regulated by the transport efficiency of the renal tubules under hormonal control.

Question 518: What is the expected Transtubular Potassium Gradient (TTKG) in a patient with hypokalemia due to extrarenal losses?

A. < 3-4 (Correct Answer)
B. 3-4
C. > 4-5
D. > 5-6

Explanation: ***< 3-4*** - A **Transtubular Potassium Gradient (TTKG)** of less than 3-4 indicates appropriate renal potassium conservation in response to hypokalemia. - This suggests that the hypokalemia is likely due to **extrarenal losses**, such as gastrointestinal losses (diarrhea, vomiting) or inadequate dietary intake, as the kidneys are working to retain potassium. *3-4* - A TTKG value in this range is typically considered indeterminate but could still point towards appropriate renal conservation if other clinical signs of extrarenal losses are present. - However, it does not as strongly confirm appropriate renal conservation as a value clearly below 3. *> 4-5* - A TTKG greater than 4-5 suggests **inappropriate renal potassium excretion** for a patient with hypokalemia. - This would indicate that the kidneys are complicit in the potassium loss, pointing towards renal causes of hypokalemia, such as **mineralocorticoid excess** or **diuretic use**. *> 5-6* - A TTKG greater than 5-6 strongly indicates significant **renal potassium wasting**. - This would be seen in conditions where the kidneys are actively secreting potassium despite hypokalemia, thereby contributing to the low potassium levels rather than conserving it.

Question 519: What is the minimum fluid urine output for neutral solute balance?

A. 300 ml
B. 750 ml
C. 500 ml
D. 400 ml (Correct Answer)

Explanation: ***400 ml*** - The kidneys must excrete approximately **600 mOsm of solutes daily** to maintain neutral solute balance. - With a maximum urine concentrating ability of **1200-1400 mOsm/L**, the minimum volume required is calculated as: 600 mOsm ÷ 1400 mOsm/L = **428 ml**. - Therefore, **400 ml** is the conventionally accepted minimum urine output for neutral solute balance. - Below this volume, even with maximal concentration, solute excretion would be inadequate. *300 ml* - **300 ml** would be insufficient to excrete the 600 mOsm daily solute load even at maximal concentration (300 × 1400 = 420 mOsm only). - This volume would lead to accumulation of solutes and **azotemia** (elevated BUN and creatinine). *500 ml* - While **500 ml** would certainly be adequate for solute excretion, it exceeds the calculated minimum of ~428 ml. - The question asks for the *minimum* volume, making **400 ml** the more precise answer according to standard textbooks. *750 ml* - **750 ml** is well above the minimum required for neutral solute balance. - This volume represents normal physiological urine output but is not the minimum threshold for maintaining solute balance.

Question 520: All should be features of a substance to measure GFR, except?

A. Freely reabsorbed (Correct Answer)
B. Not secreted by kidney
C. Freely filtered across the glomerulus membrane
D. None of the options

Explanation: ***Freely reabsorbed*** - A substance used to measure GFR should **not be reabsorbed** by the kidney tubules. If it were reabsorbed, the amount excreted in the urine would be less than the amount filtered, leading to an **underestimation of GFR**. - The ideal GFR marker is **neither reabsorbed nor secreted**, ensuring that its excretion rate directly reflects the filtration rate. *Freely filtered across the glomerulus membrane* - For a substance to accurately measure GFR, it must be **freely filtered** from the blood into the Bowman's capsule, without any restriction due to its size or charge. - This ensures that its concentration in the glomerular filtrate is the same as in the plasma, allowing for a direct calculation of the filtration rate. *Not secreted by kidney* - An ideal GFR marker should **not be secreted** by the renal tubules, as active secretion would add to the amount excreted in the urine, leading to an **overestimation of GFR**. - This property, along with not being reabsorbed, ensures that the amount of the substance appearing in the urine solely reflects the amount filtered. *None of the options* - This option is incorrect because there is a definitive feature listed among the choices that is *not* a characteristic of an ideal GFR marker. The ability to be "freely reabsorbed" is a disqualifying trait.

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

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Sodium and Water Balance

Practice Questions

Potassium Regulation

Practice Questions

Calcium and Phosphate Handling

Practice Questions

Micturition Physiology

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Renal Function Tests

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Integrative Responses to Fluid Challenges

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