Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 451: Parathormone increases calcium reabsorption by acting at -

A. Loop of Henle
B. PCT
C. Collecting duct
D. DCT (Correct Answer)

Explanation: ***DCT*** - Parathormone (PTH) primarily increases **calcium reabsorption** in the kidneys by acting on the **distal convoluted tubule (DCT)**. - This action involves stimulating the activity of **calcium channels** and pumps in the apical and basolateral membranes of DCT cells. *Loop of Henle* - The loop of Henle is mainly involved in establishing the **osmotic gradient** in the kidney and reabsorbing water and some ions like sodium and chloride, but not significantly calcium under PTH control. - While some calcium reabsorption occurs in the thick ascending limb, it is not the primary site of **PTH-mediated calcium absorption**. *PCT* - The **proximal convoluted tubule (PCT)** is the primary site for reabsorbing filtered substances like glucose, amino acids, and a large portion of sodium and water. - While about 60-70% of filtered calcium is reabsorbed here, this process is largely **PTH-independent**. *Collecting duct* - The collecting duct is mainly involved in fine-tuning water reabsorption (controlled by **ADH**) and potassium and acid-base balance. - It plays a minor role in calcium reabsorption, and its activity is not a primary target for **PTH's calcium-regulating effects**.

Question 452: Mineralocorticoid receptors are present in all of the following sites, Except:

A. Colon
B. Kidney
C. Liver (Correct Answer)
D. Hippocampus

Explanation: ***Liver*** - Among the options listed, the liver has the **least prominent and least classical expression** of mineralocorticoid receptors (MRs) compared to the other sites. - While hepatocytes do express MRs, the liver is primarily known for **metabolizing aldosterone and other steroids** rather than being a primary classical target organ for mineralocorticoid action. - The physiological significance of hepatic MRs is an area of ongoing research, with roles in gluconeogenesis and metabolic regulation being explored. *Colon* - The colon is a **classical target organ** with prominent expression of **mineralocorticoid receptors (MRs)** in epithelial cells. - MRs in the colon mediate **sodium and water absorption**, particularly in the distal colon, contributing to fluid and electrolyte balance. *Kidney* - The kidney is the **primary and most well-established target organ** for mineralocorticoids, with abundant **MRs** in the distal tubules and collecting ducts. - These receptors mediate **sodium reabsorption** and **potassium/hydrogen ion excretion**, playing a central role in blood pressure regulation and electrolyte homeostasis. *Hippocampus* - The hippocampus contains a **high concentration of mineralocorticoid receptors (MRs)**, making it an important extrarenal target. - Hippocampal MRs are involved in **cognitive functions**, **stress response**, **mood regulation**, and neuroplasticity, modulating the effects of corticosteroids on the central nervous system.

Question 453: Aldosterone acts chiefly on which of the following parts of the nephron:

A. DCT (Correct Answer)
B. Glomerulus
C. Loop of Henle
D. PCT

Explanation: ***DCT*** - Aldosterone primarily acts on the **principal cells** in the **distal convoluted tubule (DCT)** and collecting duct. - Its main roles are increasing **sodium reabsorption** and **potassium secretion** from the tubular fluid. *Glomerulus* - The glomerulus is responsible for **filtration** of blood, not hormonal regulation of electrolyte reabsorption. - It is where the initial filtrate is formed, based on pressure gradients. *Loop of Henle* - The Loop of Henle is crucial for establishing the **medullary osmotic gradient**, primarily through reabsorption of water (descending limb) and solutes (ascending limb). - It does not have significant receptors for aldosterone regulation. *PCT* - The proximal convoluted tubule (PCT) is the primary site for the **non-regulated reabsorption** of most filtered solutes, including a large percentage of sodium, glucose, and amino acids. - Its reabsorptive functions are largely independent of aldosterone.

Question 454: Which of the following substances is not typically excreted in urine under normal physiological conditions?

A. Creatinine
B. Uric acid
C. Sodium
D. HCO3- (Correct Answer)

Explanation: ***HCO3-*** - Under normal physiological conditions, almost all **filtered bicarbonate** is reabsorbed in the renal tubules to maintain the body's **acid-base balance**. - Its presence in significant amounts in urine would indicate a disorder of acid-base regulation, such as **renal tubular acidosis**. *Creatinine* - **Creatinine** is a waste product of muscle metabolism that is freely filtered by the glomeruli and **not reabsorbed** or further metabolized. - It is consistently excreted in urine, making it a useful marker for **glomerular filtration rate (GFR)**. *Uric acid* - **Uric acid** is a metabolic by-product of **purine metabolism** that is both filtered and secreted by the kidneys, although a significant portion is reabsorbed. - Excretion of uric acid in urine is normal and important for preventing its accumulation, which can lead to conditions like **gout**. *Sodium* - **Sodium** is a major electrolyte whose excretion in urine is tightly regulated by the kidneys to maintain **fluid balance** and **blood pressure**. - While most filtered sodium is reabsorbed, the amount excreted in urine varies depending on dietary intake and hormonal influences such as **aldosterone**.

Question 455: Which statement about antidiuretic hormone (ADH) is true?

A. It is synthesized in the posterior pituitary gland
B. It decreases both urine and plasma osmolality
C. It stimulates thirst
D. It increases water reabsorption in the collecting tubules and ducts (Correct Answer)

Explanation: ***It increases water reabsorption in the collecting tubules and ducts*** - **Antidiuretic hormone (ADH)**, also known as **vasopressin**, binds to V2 receptors in the **collecting ducts and tubules** of the kidney, promoting the insertion of **aquaporin-2 channels**. - This action significantly increases the **reabsorption of water** from the tubular fluid back into the bloodstream, thereby concentrating urine and conserving body water. *It is synthesized in the posterior pituitary gland* - **ADH** is actually synthesized in the **hypothalamus** (specifically in the supraoptic and paraventricular nuclei). - It is then transported down axons to the **posterior pituitary gland**, where it is stored and released. *It decreases both urine and plasma osmolality* - This is **incorrect**. **ADH** increases water reabsorption, which leads to **increased urine osmolality** (more concentrated urine) and **decreased plasma osmolality** (more dilute plasma). - Therefore, ADH causes urine osmolality to **increase** while plasma osmolality **decreases**, not both decreasing. *It stimulates thirst* - While **ADH** helps to regulate water balance, its primary direct effect is on kidney water reabsorption and systemic **vasoconstriction**. - **Thirst** is primarily stimulated by an increase in **plasma osmolality**, which also triggers ADH release, but ADH itself does not directly stimulate thirst.

Question 456: In Bartter's syndrome there is a defect in

A. Descending limb of LOH
B. Thick ascending limb of LOH (Correct Answer)
C. DCT
D. PCT

Explanation: ***Thick ascending limb of LOH*** - **Bartter's syndrome** is characterized by a genetic defect affecting the **Na-K-2Cl cotransporter (NKCC2)** located in the thick ascending limb of the loop of Henle. - This defect impairs the reabsorption of sodium, potassium, and chloride ions, leading to significant **electrolyte imbalances** such as hypokalemia, metabolic alkalosis, and hyperreninemia. *Descending limb of LOH* - The descending limb is primarily permeable to **water** due to aquaporin channels, and impermeable to solutes. - Defects in this segment are not typically associated with the electrolyte derangements seen in Bartter's syndrome. *DCT* - The **distal convoluted tubule (DCT)** is where fine-tuning of sodium and calcium reabsorption occurs, primarily through the Na-Cl cotransporter (NCC) and active calcium transport. - Defects in the DCT are characteristic of **Gitelman's syndrome**, which has similar but generally milder symptoms compared to Bartter's syndrome. *PCT* - The **proximal convoluted tubule (PCT)** is responsible for the bulk reabsorption of filtered substances, including glucose, amino acids, bicarbonate, and about 65-70% of filtered sodium. - While defects here can lead to various syndromes (e.g., Fanconi syndrome), they do not directly cause the specific electrolyte abnormalities seen in Bartter's syndrome.

Question 457: Which of the following results in increased ADH activity?

A. Hypervolemia
B. Hypertension
C. Decrease osmolarity
D. Hypovolemia (Correct Answer)

Explanation: ***Hypovolemia*** - **Hypovolemia** (decreased blood volume) is a potent stimulus for antidiuretic hormone (ADH) release, as the body attempts to conserve water and increase blood volume. - Reduced **stretch receptor activity** in the atria and great vessels due to decreased blood volume signals the posterior pituitary to release ADH. *Hypervolemia* - **Hypervolemia** (increased blood volume) would lead to a *decrease* in ADH activity, as the body tries to excrete excess water to normalize blood volume. - Increased stretch receptor activity in the atria inhibits ADH release. *Hypertension* - **Hypertension** (high blood pressure) generally *reduces* ADH secretion because the increased stretch on baroreceptors signals to decrease fluid volume. - This typically promotes diuresis rather than water retention. *Decrease osmolarity* - A **decrease in plasma osmolarity** (more dilute blood) inhibits ADH release, as the body aims to excrete excess water to bring osmolarity back to normal. - **Osmoreceptors** in the hypothalamus are sensitive to changes in plasma osmolarity and are the primary regulators of ADH secretion.

Question 458: Which causes raised angiotensin in blood?

A. Raised cardiac output
B. Increased sympathetic tone
C. Increased blood volume
D. Decreased blood pressure (Correct Answer)

Explanation: ***Decreased blood pressure*** - A decrease in blood pressure is the **primary physiological trigger** that signals the kidneys to release **renin**, initiating the **renin-angiotensin-aldosterone system (RAAS)**. - Renal baroreceptors in the juxtaglomerular apparatus sense decreased renal perfusion pressure and stimulate renin release. - Renin converts **angiotensinogen** to **angiotensin I**, which is then converted to **angiotensin II** (the active form) by **angiotensin-converting enzyme (ACE)**. - This represents the most direct and important mechanism for raising angiotensin levels in response to hemodynamic changes. *Raised cardiac output* - **Increased cardiac output** generally leads to **increased blood pressure**, which would suppress renin release and reduce angiotensin levels. - The body's homeostatic mechanisms aim to lower blood pressure in response to increased cardiac output, not raise angiotensin. - This has the opposite effect on the RAAS system. *Increased sympathetic tone* - While **increased sympathetic tone does stimulate renin release** via β1-adrenergic receptors on juxtaglomerular cells, it is typically a **secondary mechanism** that occurs in response to decreased blood pressure. - Sympathetic stimulation is one of three major stimuli for renin release, but in physiological terms, it usually acts as part of the compensatory response to hypotension rather than as an independent primary cause. - The question asks for the cause of raised angiotensin, and decreased blood pressure is the more direct and primary trigger. *Increased blood volume* - **Increased blood volume** results in **elevated blood pressure**, which would suppress renin release and consequently lower angiotensin levels. - Atrial natriuretic peptide (ANP) is released in response to increased blood volume, which inhibits renin secretion. - This has the opposite effect on angiotensin levels.

Question 459: Calcitriol is formed in:

A. Glomerulus
B. Bowman's capsule
C. PCT (Correct Answer)
D. DCT

Explanation: ***PCT*** - The final step in calcitriol (active vitamin D) synthesis, 1-alpha hydroxylation, primarily occurs in the **proximal convoluted tubule (PCT)** cells of the kidney. - This enzymatic step converts **25-hydroxyvitamin D** into the potent hormone **1,25-dihydroxyvitamin D (calcitriol)**, which regulates calcium and phosphate homeostasis. *Glomerulus* - The **glomerulus** is primarily responsible for **filtering blood** to form ultrafiltrate, not for hormone synthesis. - While vitamin D precursors are filtered, the enzymatic conversion to calcitriol does not occur here. *Bowman's capsule* - **Bowman's capsule** surrounds the glomerulus and collects the filtered fluid, acting as a passive receiver. - It plays no direct role in the synthesis or metabolism of vitamin D. *DCT* - The **distal convoluted tubule (DCT)** is involved in fine-tuning reabsorption of ions like calcium and sodium, responding to hormones. - It is not the primary site for the **1-alpha hydroxylation** required for calcitriol synthesis.

Question 460: Which of the following does not have a Tm value?

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

Explanation: ***Creatinine*** - **Creatinine** is freely filtered at the glomerulus but is **not reabsorbed** by the renal tubules. - Since there is **no reabsorption mechanism** for creatinine, there is no tubular maximum (Tm) for reabsorption. - A small amount of creatinine is secreted in the proximal tubule, but secretion mechanisms are distinct from the Tm concept which applies to reabsorption. - This makes creatinine an excellent marker for **glomerular filtration rate (GFR)** assessment. *Glucose* - **Glucose** has a well-defined **tubular maximum (Tm)** of approximately **375 mg/min** in adults. - It is reabsorbed via specific carrier proteins (**SGLT2** in early proximal tubule, **SGLT1** in late proximal tubule). - When plasma glucose exceeds the renal threshold (~180 mg/dL), the carriers become saturated and glucose appears in urine (glucosuria). *Urea* - **Urea** undergoes passive reabsorption driven by concentration gradients, particularly in the proximal tubule (~50%) and collecting duct. - While primarily passive, urea transporters (UT-A1, UT-A3) facilitate its movement and can exhibit **saturation kinetics**. - Some sources consider urea to have a form of Tm, though it's less clearly defined than glucose. *Na+* - **Na+** reabsorption occurs via multiple carrier-mediated mechanisms including **Na-K-ATPase**, Na-H exchangers, and various cotransporters. - These carrier systems **do exhibit saturation kinetics** and can be considered to have Tm characteristics, particularly in the proximal tubule where ~65% of filtered Na+ is reabsorbed. - However, Na+ handling is also heavily regulated by hormones (aldosterone, ANP) and hemodynamic factors, making its Tm less clinically apparent than glucose.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

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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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