Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 551: Which of the following is the mode of action of vasopressin?

A. Increases water permeability in the collecting duct (Correct Answer)
B. Promotes water reabsorption at proximal convoluted tubule
C. Stimulates water secretion at loop of Henle
D. Decreases sodium reabsorption in the distal tubule

Explanation: ***Increases water permeability in the collecting duct*** - **Vasopressin**, also known as **antidiuretic hormone (ADH)**, primarily acts on the **collecting ducts** of the kidney. - It binds to **V2 receptors** on the basolateral membrane, activating the **cAMP-PKA pathway**, which increases the insertion of **aquaporin-2 (AQP2)** water channels into the apical membrane, significantly enhancing water permeability and subsequent reabsorption. *Promotes water reabsorption at proximal convoluted tubule* - The **proximal convoluted tubule (PCT)** is responsible for the reabsorption of approximately **65% of filtered water**, but this process is **constitutive** and **not regulated by vasopressin**. - Water reabsorption in the PCT follows the osmotic gradient created by active sodium reabsorption, independent of hormonal control. *Stimulates water secretion at loop of Henle* - The **loop of Henle** is primarily involved in establishing an **osmotic gradient** in the renal medulla, essential for concentrating urine. - The **descending limb** is permeable to water (allowing passive reabsorption, not secretion), while the **ascending limb** is impermeable to water. - Vasopressin does not stimulate water secretion in any part of the nephron. *Decreases sodium reabsorption in the distal tubule* - Vasopressin **does not decrease sodium reabsorption**; in fact, it may indirectly enhance it. - The primary regulators of sodium reabsorption in the distal tubule are **aldosterone** (increases reabsorption) and **atrial natriuretic peptide (ANP)** (decreases reabsorption). - Vasopressin's main action is on water permeability, not direct sodium handling in the distal tubule.

Question 552: Which of the following statements is true about the ascending limb of the loop of Henle?

A. Receives hypotonic saline
B. Impermeable to water (Correct Answer)
C. Respond to ADH
D. Maximum secretion of H+ ions

Explanation: ***Impermeable to water*** - The ascending limb of the loop of Henle is **impermeable to water**, which is its defining characteristic. - It actively reabsorbs **Na+, K+, and Cl-** via the **NKCC2 transporter** (target of loop diuretics like furosemide). - This solute reabsorption without water reabsorption makes the tubular fluid **hypotonic** and contributes to the **medullary osmotic gradient**. - This is a key feature distinguishing it from the water-permeable descending limb. *Receives hypotonic saline* - The ascending limb receives fluid from the descending limb that is **hypertonic** (concentrated), not hypotonic. - The descending limb is permeable to water, so water exits and the fluid becomes concentrated. - The fluid **leaving** the ascending limb becomes hypotonic due to solute reabsorption without water. *Maximum secretion of H+ ions* - Maximum **H+ secretion** occurs in the **proximal tubule** and **collecting duct** (intercalated cells). - The ascending limb's primary function is **solute reabsorption** (Na+, K+, Cl-), not acid-base regulation. - The thick ascending limb does contribute to acid-base balance but is not the primary site. *Respond to ADH* - The ascending limb does **NOT respond to ADH** (Antidiuretic Hormone). - ADH acts primarily on the **collecting ducts** and to a lesser extent the **distal convoluted tubule**. - The ascending limb lacks ADH receptors and remains impermeable to water regardless of ADH levels.

Question 553: Where does potassium reabsorption primarily occur in the kidney?

A. Primarily in the PCT (Correct Answer)
B. In the glomerulus
C. Limited to the descending limb of Henle
D. In the collecting duct through secretion

Explanation: ***Primarily in the PCT*** - The **proximal convoluted tubule (PCT)** reabsorbs the majority (65-70%) of filtered potassium, making it the **primary site** of potassium reabsorption. - This reabsorption occurs primarily through **paracellular routes** along with water and sodium reabsorption (solvent drag). - The PCT plays a crucial role in maintaining overall **potassium homeostasis** by retrieving most of the filtered load before the filtrate reaches distal segments. *In the glomerulus* - The **glomerulus** is responsible for **filtration** of blood, not reabsorption of substances. - All small solutes, including potassium, are freely filtered across the **glomerular capillaries** into Bowman's capsule. - Filtration is the first step; reabsorption occurs in tubular segments. *In the collecting duct through secretion* - The **collecting duct** is primarily involved in **potassium secretion** into the urine, regulated by aldosterone, to fine-tune potassium balance. - While some reabsorption can occur via intercalated cells, the net effect in the collecting duct is **secretion**, not reabsorption. - This is critical for regulating final urinary K+ excretion. *Limited to the descending limb of Henle* - The **descending limb of the loop of Henle** is primarily permeable to **water** but relatively impermeable to solutes. - **No significant potassium reabsorption** occurs in the descending limb. - Significant K+ reabsorption also occurs in the thick ascending limb (20-25% of filtered load), but the PCT remains the primary site.

Question 554: What is the filtration characteristic of neutral substances that are 6 nm in size?

A. Not filtered
B. Not freely filtered (Correct Answer)
C. Filtered based on size
D. None of the options

Explanation: ***Not freely filtered*** - At **6 nm**, a neutral substance experiences **significant size restriction** at the glomerular filtration barrier - The **effective pore size** of the glomerular basement membrane and slit diaphragms allows **partial but restricted passage** at this size - Filtration coefficient (θ) at 6 nm is approximately **0.2-0.5**, meaning only 20-50% is filtered compared to small freely filtered molecules - The term **"not freely filtered"** accurately describes this substantial size-based restriction *Filtered based on size* - While technically all filtration is based on size and charge, this term is **too vague** and doesn't convey the **degree of restriction** - At 6 nm, the key point is that filtration is **significantly impaired**, not just "size-dependent" - This option fails to distinguish between molecules that pass easily vs. those that are substantially restricted *Not filtered* - This is too absolute; **some filtration does occur** at 6 nm for neutral molecules - Complete exclusion applies to molecules **>8 nm** or large plasma proteins like albumin (~7 nm diameter with negative charge) - A 6 nm neutral substance has approximately **20-50% filterability**, so it cannot be described as "not filtered" *None of the options* - Incorrect, as **"Not freely filtered"** is the precise physiological term for a 6 nm neutral substance - This accurately reflects the **partial restriction** based on molecular size relative to the glomerular pore dimensions

Question 555: Minimum urinary output required to excrete the end products of protein metabolism is

A. 200 mL/24 hours
B. 300 mL/24 hours
C. 400 mL/24 hours (Correct Answer)
D. 500 mL/24 hours

Explanation: ***400 mL/24 hours*** - This volume is considered the **obligatory urine output** needed to excrete the daily solute load, primarily derived from **protein metabolism**, preventing the accumulation of toxic waste products. - This minimum ensures that the kidneys can adequately perform their excretory function without causing **azotemia** or other metabolic imbalances. *200 mL/24 hours* - This volume is generally too low to effectively excrete the daily metabolic waste products from protein breakdown, often leading to **oliguria** and potential **uremic toxicity**. - A persistent urine output this low would suggest **acute kidney injury** or severe dehydration, where metabolic waste accumulation is likely. *300 mL/24 hours* - While better than 200 mL, 300 mL per 24 hours is still generally insufficient to excrete the typical daily solute load produced by protein metabolism for an average adult, risking **waste product accumulation**. - This level is closer to the definition of **oliguria**, indicating inadequate renal function or significant fluid deficit. *500 mL/24 hours* - This volume is generally sufficient for excreting metabolic waste products and is often considered a normal lower limit for fluid balance, but **400 mL/24 hours** is the established physiological minimum for solute excretion. - While 500 mL/24 hours is adequate, the question asks for the *minimum* required, and 400 mL represents the **absolute lowest functional threshold**.

Question 556: Which of the following statements is MOST true regarding water reabsorption in the nephron?

A. Facultative reabsorption primarily occurs in the collecting ducts.
B. The bulk of water reabsorption occurs in the proximal tubule secondary to Na+ reabsorption. (Correct Answer)
C. Obligatory reabsorption occurs regardless of hydration state.
D. Water reabsorption can vary significantly depending on the body's hydration needs.

Explanation: ***The bulk of water reabsorption occurs in the proximal tubule secondary to Na+ reabsorption.*** - Approximately 65-70% of filtered water is reabsorbed in the **proximal tubule**, largely driven by the active transport of **Na+**, which creates an osmotic gradient. - This process is **obligatory**, meaning it occurs regardless of the body's hydration status, and is essential for maintaining fluid balance. *Facultative reabsorption primarily occurs in the collecting ducts.* - While facultative water reabsorption, **regulated by ADH**, does occur in the **collecting ducts**, this statement is not "most true" because it overlooks the quantitative significance of the proximal tubule. - The collecting ducts are responsible for fine-tuning water reabsorption to match the body's hydration needs, but only a smaller, variable amount compared to the proximal tubule. *Obligatory reabsorption occurs regardless of hydration state.* - This statement is true, but it is not the MOST true statement compared to the option highlighting the bulk reabsorption in the proximal tubule. **Obligatory reabsorption** primarily occurs in the **proximal convoluted tubule** and **loop of Henle**. - It is a constant process that recovers a large, fixed percentage of filtered water, essential for basic volume maintenance **independent of ADH**. *Water reabsorption can vary significantly depending on the body's hydration needs.* - This statement is true, specifically referring to **facultative water reabsorption**, which is regulated by **antidiuretic hormone (ADH)** in the collecting ducts. - However, this variation is only for about 10-20% of total reabsorption, while the *bulk* of reabsorption is constant and occurs in the **proximal tubule**.

Question 557: Glomerular filtrate contains all except what?

A. Glucose
B. Sodium
C. HCO3
D. Albumin (Correct Answer)

Explanation: ***Albumin*** - **Albumin** is a large protein that normally does not pass through the **glomerular filtration barrier** due to its size and negative charge. - Its presence in the urine (**albuminuria**) indicates **glomerular damage**. *HCO3* - **Bicarbonate ions (HCO3-)** are small enough to be freely filtered at the glomerulus. - They are crucial for **acid-base balance** and are extensively reabsorbed in the renal tubules. *Glucose* - **Glucose** is a small molecule that is freely filtered by the glomerulus. - Under normal conditions, almost all filtered glucose is reabsorbed in the **proximal tubule**. *Sodium* - **Sodium ions (Na+)** are small and are readily filtered across the glomerulus. - They play a vital role in **fluid balance** and are actively reabsorbed along the entire tubule.

Question 558: What is the percentage of Na+ reabsorbed from the ascending loop of Henle via the Na+–K+–2Cl– symporter?

A. 10% Na+ – H+ exchange
B. 50% Na+ - K+ - 2Cl– symporter
C. 30% Na+ – Cl– symporter
D. 20% Na+ – K+ – 2Cl– symporter (Correct Answer)

Explanation: ***20% Na+ – K+ – 2Cl– symporter*** - Approximately **20-25%** of filtered Na+ is reabsorbed in the **thick ascending limb of the loop of Henle** primarily via the **Na+-K+-2Cl- symporter (NKCC2)**. - This symporter is crucial for concentrating the urine and creating the **medullary osmotic gradient**. *10% Na+ – H+ exchange* - **Na+-H+ exchange** (NHE3) is the primary mechanism for Na+ reabsorption in the **proximal convoluted tubule**, where about **65-70%** of filtered Na+ is reabsorbed. - This exchange is also vital for **bicarbonate reabsorption** and **pH regulation**, not the primary Na+ reabsorption in the ascending loop of Henle. *50% Na+ - K+ - 2Cl– symporter* - Reabsorption of **50% of Na+** by the Na+-K+-2Cl- symporter would be an **overestimation** of its role in the thick ascending limb. - While it's a significant route, the total Na+ reabsorption in this segment is closer to **20-25%**. *30% Na+ – Cl– symporter* - The **Na+-Cl- symporter** is the main mechanism for Na+ reabsorption in the **distal convoluted tubule**, where approximately **5-10%** of filtered Na+ is reabsorbed. - This transport mechanism is responsible for the reabsorption of Na+ and Cl- without significant water reabsorption, making this segment important for **further diluting the filtrate**.

Question 559: The site of action of vasopressin is

A. Descending limb of the loop of Henle
B. Proximal tubule
C. Distal tubule and collecting duct (Correct Answer)
D. Ascending limb of the loop of Henle

Explanation: ***Distal tubule*** - Vasopressin, also known as **antidiuretic hormone (ADH)**, primarily acts on the collecting ducts and the late distal convoluted tubule. - Its main function is to increase the **reabsorption of water** by inserting **aquaporin-2 channels** into the apical membrane of principal cells. *Ascending limb of the loop of Henle* - This segment is largely **impermeable to water** regardless of hormonal influence, focusing on active reabsorption of solutes. - Its primary role is to dilute the tubular fluid and reabsorb ions like **Na+, K+, and Cl-**. *Descending limb of the loop of Henle* - This limb is freely permeable to water but largely impermeable to solutes, allowing water to exit the tubule due to the **medullary osmotic gradient**. - Vasopressin does not significantly influence water permeability in this segment. *Proximal tubule* - The proximal tubule reabsorbs a large percentage (about 65-70%) of filtered water and solutes in a relatively unregulated manner. - Its function is largely independent of vasopressin, which acts further down the nephron to fine-tune water reabsorption.

Question 560: In renal failure, what is the primary cause of metabolic acidosis?

A. Use of diuretics
B. Loss of HCO3-
C. Increased H+ production
D. Decreased excretion of acids (Correct Answer)

Explanation: ***Decreased excretion of acids*** - In **renal failure**, the kidneys lose their ability to effectively excrete metabolic acid byproducts, leading to their accumulation in the body. - This accumulation of acids, such as **sulfates**, **phosphates**, and **urea**, consumes bicarbonate buffers, resulting in metabolic acidosis. *Increased H+ production* - While overproduction of **H+ ions** can cause acidosis, like in **ketoacidosis** or **lactic acidosis**, it's not the primary underlying mechanism in most cases of renal failure. - The problem in renal failure is primarily one of **impaired elimination**, not excessive generation, of acids. *Loss of HCO3-* - Loss of **bicarbonate (HCO3-)** can occur in conditions like severe diarrhea or renal tubular acidosis, but it's not the primary cause of metabolic acidosis in general renal failure. - In renal failure, decreased **ammoniagenesis** and impaired reabsorption of bicarbonate can contribute, but the main driver is reduced acid excretion. *Use of diuretics* - The use of **diuretics** (especially loop or thiazide diuretics) typically causes **metabolic alkalosis** due to increased potassium and hydrogen ion excretion, rather than acidosis. - Some diuretics, like **carbonic anhydrase inhibitors**, can cause a mild metabolic acidosis, but this is less common and not the primary cause of renal failure-associated acidosis.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

Tubular Reabsorption and Secretion

Practice Questions

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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