Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 481: A 60-year-old male presents with severe dehydration after a marathon. Which of the following compensatory mechanisms is most likely to occur in his body?

A. Decreased renin secretion
B. Decreased ADH secretion
C. Increased aldosterone secretion (Correct Answer)
D. Decreased aldosterone secretion

Explanation: ***Increased aldosterone secretion*** - **Dehydration** leads to decreased **blood volume** and **blood pressure**, which stimulates the **renin-angiotensin-aldosterone system (RAAS)**. - **Aldosterone** specifically promotes **sodium reabsorption** and **potassium excretion** in the kidneys, which helps to increase water retention and restore blood volume. *Decreased renin secretion* - **Dehydration** would actually trigger an **increase** in **renin secretion** due to reduced **renal perfusion** and activation of the baroreceptors. - This increase in renin is part of the body's compensatory mechanism to restore blood volume and pressure. *Decreased ADH secretion* - **Antidiuretic hormone (ADH)**, also known as **vasopressin**, production would **increase** significantly in response to **dehydration** and increased plasma osmolality. - ADH promotes water reabsorption in the kidneys to reduce water loss and conserve body fluids. *Decreased aldosterone secretion* - **Dehydration** causes the body to activate mechanisms to conserve water and electrolytes, not decrease them. - Therefore, **aldosterone secretion** would **increase** to help reabsorb sodium and water, counteracting the effects of dehydration.

Question 482: A 45-year-old male presents with polyuria and polydipsia. Which renal process is most likely to be impaired?

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

Explanation: ***Collecting duct water reabsorption*** - **Polyuria** and **polydipsia** are classic symptoms of **diabetes insipidus**, a condition characterized by impaired water reabsorption in the collecting ducts. - This impairment can be due to either a deficiency in antidiuretic hormone (ADH) production (central diabetes insipidus) or a lack of renal response to ADH (nephrogenic diabetes insipidus). *Proximal tubule sodium reabsorption* - Impaired proximal tubule sodium reabsorption would primarily lead to **sodium wasting**, potentially causing **hypovolemia** and **hyponatremia**, rather than isolated polyuria and polydipsia. - While overall fluid balance would be affected, the primary defect isn't the specific cause of profound polyuria and thirst. *Loop of Henle water reabsorption* - The **Loop of Henle's** primary role in water reabsorption is crucial for concentrating urine and maintaining the **medullary osmotic gradient**. - Impairment here (e.g., due to loop diuretics) would cause polyuria, but the collecting duct is the final and most significant site for ADH-regulated water reabsorption, making its impairment a more direct cause of the symptoms described. *Distal tubule calcium reabsorption* - The **distal tubule** is important for **calcium reabsorption**, which is regulated by parathyroid hormone (PTH). - Impairment in this process would primarily lead to **hypercalciuria** and potential **hypocalcemia**, not directly causing polyuria and polydipsia as the main symptoms.

Question 483: What is a common physiological response observed in acute renal failure?

A. Activation of antidiuretic hormone
B. Increased urine output
C. Decreased glomerular filtration rate (Correct Answer)
D. Increased secretion of aldosterone

Explanation: ***Decreased glomerular filtration rate*** - A hallmark of **acute kidney injury (AKI)** is a significant reduction in the **glomerular filtration rate (GFR)**, indicating impaired renal function. - This reduction in GFR leads to the accumulation of waste products in the blood, such as **urea** and **creatinine**. *Increased secretion of aldosterone* - While fluid and electrolyte imbalances occur in AKI, **increased aldosterone secretion** is more typically associated with conditions leading to decreased effective circulating volume, which ultimately tries to preserve volume. - In AKI, the primary issue is the kidney's inability to filter, not necessarily a systemic drive to conserve sodium and water via increased aldosterone. *Increased urine output* - **Oliguria** (decreased urine output) or **anuria** (absence of urine output) is a common presentation of AKI, especially in intrinsic renal failure. - An increase in urine output would be contradictory to the definition and pathophysiology of acute renal failure (AKI). *Activation of antidiuretic hormone* - **Antidiuretic hormone (ADH)** activation primarily regulates water reabsorption in response to increased plasma osmolality or decreased blood volume. - While fluid balance is disrupted in AKI, the core problem is renal filtration failure, and ADH activation is a secondary response, not the defining physiological change of AKI itself.

Question 484: In chronic kidney disease, the ability to concentrate urine is impaired. Which part of the nephron is primarily responsible for this function?

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

Explanation: ***Loop of Henle*** - The **Loop of Henle** is crucial for establishing the **medullary osmotic gradient** through its countercurrent multiplier mechanism. - This gradient allows the **collecting ducts** to reabsorb water, thereby concentrating urine under the influence of **ADH**. *Proximal convoluted tubule* - The **proximal convoluted tubule** primarily reabsorbs most filtered **solutes** (e.g., glucose, amino acids, bicarbonate, and salts) and water in an **isosmotic** fashion. - It does not play a significant role in creating a concentrated urine. *Distal convoluted tubule* - The **distal convoluted tubule** is involved in fine-tuning the reabsorption of **sodium**, **calcium**, and **bicarbonate**, and secretion of **potassium** and **hydrogen ions**. - Its reabsorption of water is limited and less impactful on final urine concentration compared to the Loop of Henle and collecting duct. *Collecting duct* - While the **collecting duct** is where the final urine concentration occurs by reabsorbing water, it *depends* on the **osmotic gradient** established by the Loop of Henle. - Without the proper function of the Loop of Henle, the collecting duct cannot effectively concentrate urine, even in the presence of **ADH**.

Question 485: In a patient experiencing dehydration, which hormone is primarily responsible for conserving body water?

A. Aldosterone
B. Antidiuretic hormone (ADH) (Correct Answer)
C. Cortisol
D. Growth hormone

Explanation: ***Antidiuretic hormone (ADH)*** - **ADH**, also known as **vasopressin**, is released in response to increased plasma osmolality or decreased blood volume (as seen in **dehydration**). - It acts on the **renal collecting ducts** to increase water reabsorption, thereby **conserving body water** and concentrating urine. *Aldosterone* - **Aldosterone** primarily promotes **sodium reabsorption** and potassium excretion in the kidneys. - While it contributes to fluid volume regulation by drawing water along with sodium, its main role is not direct water conservation but rather **electrolyte balance**. *Cortisol* - **Cortisol** is a glucocorticoid involved in stress response, metabolism, and **immune function**. - It has a minor mineralocorticoid activity but is not a primary regulator of **water balance** in dehydration. *Growth hormone* - **Growth hormone** is essential for growth, cell reproduction, and regeneration. - It does not play a direct or significant role in the acute regulation of **water balance** during dehydration.

Question 486: What is the primary effect of aldosterone on the kidneys?

A. Increase potassium secretion
B. Increase sodium reabsorption (Correct Answer)
C. Decrease bicarbonate reabsorption
D. Decrease chloride reabsorption

Explanation: ***Increase sodium reabsorption*** - Aldosterone's primary role is to promote **sodium reabsorption** in the principal cells of the collecting ducts and distal tubules of the kidneys. - This action is crucial for maintaining **extracellular fluid volume** and blood pressure. *Increase potassium secretion* - While aldosterone does increase **potassium secretion** into the tubular lumen, this is a secondary effect linked to sodium reabsorption to maintain electrochemical neutrality. - The initial and most direct action is on sodium transport, which then facilitates potassium excretion. *Decrease bicarbonate reabsorption* - Aldosterone does not primarily decrease bicarbonate reabsorption; in fact, it can indirectly influence acid-base balance by promoting **hydrogen ion secretion** by intercalated cells, which can affect bicarbonate levels. - This is not a direct or primary effect on bicarbonate reabsorption. *Decrease chloride reabsorption* - Aldosterone primarily causes an increase in **sodium reabsorption**, and chloride often follows sodium passively to maintain electrical neutrality, meaning chloride reabsorption would tend to increase rather than decrease. - It does not directly inhibit chloride reabsorption.

Question 487: What is the primary function of the juxtaglomerular apparatus in the kidney?

A. Regulate glomerular filtration rate (Correct Answer)
B. Secrete erythropoietin
C. Reabsorb sodium
D. Concentrate urine

Explanation: ***Regulate glomerular filtration rate*** - The **juxtaglomerular apparatus (JGA)**, comprising the **macula densa** and **juxtaglomerular cells**, is crucial for regulating blood flow to the glomerulus and maintaining a stable **glomerular filtration rate (GFR)**. - The **macula densa** senses **sodium chloride concentration** in the distal tubule and signals the **juxtaglomerular cells** to release **renin**, which ultimately affects **afferent arteriolar tone** and GFR. *Secrete erythropoietin* - While the kidneys do produce **erythropoietin (EPO)**, it is primarily secreted by **interstitial fibroblasts** in the renal cortex and outer medulla, not the JGA. - EPO's main function is to stimulate **red blood cell production** in the bone marrow. *Reabsorb sodium* - Sodium reabsorption occurs throughout various segments of the **renal tubule**, notably the **proximal tubule**, **loop of Henle**, and **distal tubule**, not primarily by the JGA itself. - The JGA's role concerning sodium is in **sensing its concentration** to regulate GFR, not direct large-scale reabsorption. *Concentrate urine* - Urine concentration is primarily achieved by the **loop of Henle**, collecting ducts, and the **vasa recta**, which establish and maintain an **osmotic gradient** in the renal medulla. - The JGA is not directly involved in the process of water reabsorption for urine concentration.

Question 488: What is the primary action of antidiuretic hormone (ADH)?

A. Increase urine production
B. Decrease blood pressure
C. Reduce urine production (Correct Answer)
D. Increase blood pressure

Explanation: ***Reduce urine production*** - **Antidiuretic hormone (ADH)**, also known as **vasopressin**, primarily acts on the renal collecting ducts to increase water reabsorption, thus concentrating the urine and **reducing its volume**. - This action helps to conserve body water and maintain **fluid balance** and **osmolarity**. *Increase urine production* - This is the opposite effect of ADH; increasing urine production would lead to **dehydration**, which ADH works to prevent. - Diuretics, not ADH, are medications or substances that **increase urine flow**. *Decrease blood pressure* - While ADH is also known as **vasopressin** due to its vasoconstrictive properties at high concentrations, its primary role in physiological conditions is water reabsorption. - **Vasoconstriction** would typically lead to an *increase* in blood pressure, not a decrease. *Increase blood pressure* - Although ADH can cause **vasoconstriction** at high doses, leading to an increase in blood pressure, this is considered a secondary effect and not its primary function in maintaining **body fluid homeostasis**. - The primary role of ADH is to regulate water reabsorption in the kidneys.

Question 489: Which physiological process is increased by both the dilation of afferent arterioles and decreased plasma colloid osmotic pressure?

A. Systemic blood pressure
B. Renal tubular reabsorption
C. Urine osmolarity
D. Glomerular filtration rate (Correct Answer)

Explanation: ***Glomerular filtration rate*** - **Dilation of afferent arterioles** increases **glomerular hydrostatic pressure (PGC)** and **renal plasma flow**, thereby increasing **glomerular filtration rate (GFR)**. - A **decrease in plasma colloid osmotic pressure (πGC)** reduces the opposing force to filtration in the glomerulus, leading to an **increased net filtration pressure** and ultimately a higher **GFR**. - **GFR is the only process increased by BOTH factors** - while afferent dilation also increases renal plasma flow, decreased plasma colloid osmotic pressure does not affect RPF (it only affects filtration pressure across the glomerular membrane). *Systemic blood pressure* - While afferent arteriole dilation can affect renal blood flow, it does not directly or universally lead to an increase in **systemic blood pressure**. - **Systemic blood pressure** is regulated by much broader mechanisms involving cardiac output and total peripheral resistance, not just intrarenal hemodynamics. - Decreased plasma colloid osmotic pressure would not increase systemic blood pressure. *Renal tubular reabsorption* - Increased GFR often leads to a higher filtered load, which may indirectly influence reabsorption, but the direct effect of these two factors is not to increase **tubular reabsorption**. - Reabsorption is primarily regulated by hormones like **aldosterone** and **ADH**, and specific transport mechanisms in the tubules. - Neither factor directly increases reabsorption rates. *Urine osmolarity* - **Urine osmolarity** is largely determined by the reabsorption of water, which is regulated by ADH and the medullary concentration gradient, not directly increased by both afferent dilation and decreased plasma colloid osmotic pressure. - An increased GFR might lead to a higher flow rate through the tubules, potentially *reducing* the time for water reabsorption and thus *decreasing* urine osmolarity under certain conditions.

Question 490: A 65-year-old male with chronic kidney disease has elevated blood urea nitrogen (BUN) levels. Which physiological mechanism is primarily responsible for this elevation?

A. Decreased glomerular filtration rate (Correct Answer)
B. Increased renal tubular reabsorption
C. Increased protein catabolism
D. Decreased renal tubular secretion

Explanation: ***Decreased glomerular filtration rate*** - A **decreased GFR** means the kidneys are less efficient at filtering waste products, such as **urea**, from the blood, leading to its accumulation and elevated BUN levels. - In chronic kidney disease, the progressive loss of nephron function directly impairs **glomerular filtration**, making it the primary reason for increased BUN. *Increased renal tubular reabsorption* - While some **urea** is reabsorbed in the tubules, an increase in this process is typically a **secondary compensatory mechanism** in states of volume depletion, not the primary cause of chronically elevated BUN in kidney disease. - Furthermore, if **GFR is severely compromised**, even increased reabsorption would not account for the significant elevation seen. *Increased protein catabolism* - **Increased protein catabolism** (e.g., due to severe illness, starvation, or corticosteroid use) would indeed increase **urea production**, but it's not the primary underlying physiological mechanism in the context of chronic kidney disease itself. - The elevated BUN in CKD primarily reflects a **failure of excretion**, rather than an overproduction of urea. *Decreased renal tubular secretion* - **Urea** is primarily cleared by **glomerular filtration** and, to a lesser extent, reabsorbed in the tubules; **active tubular secretion** of urea is not a significant physiological mechanism for its excretion. - Therefore, a decrease in this non-existent or minimal process would not be the cause of **elevated BUN**.

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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