Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 471: In a patient with severe dehydration, which of the following compensatory mechanisms work together to restore blood volume and maintain hemodynamic stability?

A. Sympathetic activation
B. ADH release
C. Increased renin secretion
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - In cases of severe dehydration, a coordinated response involving multiple compensatory mechanisms is crucial for restoring **blood volume** and maintaining **hemodynamic stability**. - No single mechanism is sufficient; their combined effects lead to **vasoconstriction**, **fluid retention**, and **increased cardiac output**. *Sympathetic activation* - Leads to **vasoconstriction** of peripheral vessels, increasing **vascular resistance** and shunting blood to vital organs. - Also increases **heart rate** and **contractility**, temporarily sustaining blood pressure and perfusion. *ADH release* - **Antidiuretic hormone (ADH)** increases water reabsorption in the **renal collecting ducts**, reducing urine output and conserving body fluid. - This helps to directly increase **circulating blood volume** by preventing further fluid loss. *Increased renin secretion* - **Renin** initiates the **renin-angiotensin-aldosterone system (RAAS)**, leading to the production of **angiotensin II** and **aldosterone**. - **Angiotensin II** is a potent vasoconstrictor, while **aldosterone** promotes sodium and water reabsorption in the kidneys, both contributing to volume restoration.

Question 472: Erythropoietin is secreted by which of the following organs?

A. Muscle
B. Kidney (Correct Answer)
C. Liver
D. Heart

Explanation: ***Kidney*** - The **kidneys** are the primary site of erythropoietin production in adults, particularly the **peritubular interstitial cells**. - Erythropoietin's main function is to stimulate **red blood cell production** in the bone marrow in response to hypoxia. *Muscle* - Muscles are involved in movement and metabolism but do not produce **erythropoietin**. - They primarily store glycogen and generate force through contraction. *Liver* - The liver produces erythropoietin during **fetal development** but contributes minimally to its production in adulthood. - Its main functions include metabolism, detoxification, and protein synthesis. *Heart* - The heart is responsible for **pumping blood** throughout the body and does not produce **erythropoietin**. - It primarily consists of cardiac muscle tissue.

Question 473: Where is the main site of sodium reabsorption in the nephron?

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

Explanation: ***Proximal tubule*** - The **proximal tubule** reabsorbs approximately **65-70% of filtered sodium**, making it the primary site of sodium reabsorption. - This high rate of reabsorption is crucial for maintaining overall **fluid and electrolyte balance**. *Collecting duct* - The **collecting duct** reabsorbs a smaller, regulated amount of sodium, typically **2-5%**, under hormonal control by **aldosterone**. - Its main roles include **fine-tuning** water and electrolyte balance. *Loop of Henle* - The **loop of Henle** reabsorbs approximately **25%** of filtered sodium, primarily in its **thick ascending limb**. - Its main function is to create a **medullary osmotic gradient** for concentrated urine. *Distal tubule* - The **distal tubule** reabsorbs about **5%** of filtered sodium, which is also influenced by hormones like **aldosterone**. - It plays a role in **acid-base balance** and diluting or concentrating urine.

Question 474: A 40-year-old woman with chronic kidney disease presents with hyperkalemia. Which of the following represents the immediate effect of hyperkalemia on membrane physiology?

A. Depolarization of resting membrane potential (Correct Answer)
B. Prolonged action potential duration
C. Decreased skeletal muscle contractility
D. Decreased cardiac excitability

Explanation: ***Depolarization of resting membrane potential*** - Hyperkalemia leads to an increase in extracellular potassium, which **decreases the transmembrane potassium gradient**. - This shifts the **resting membrane potential (RMP) toward zero (less negative)**, representing **depolarization** of the cell membrane. - This is the **immediate and direct effect** of elevated extracellular potassium on all excitable cells, based on the **Nernst equation for potassium**. *Prolonged action potential duration* - Hyperkalemia typically **shortens the action potential duration** in most excitable cells, not prolongs it. - This is primarily due to the increased potassium conductance, which hastens **repolarization**. *Decreased skeletal muscle contractility* - While this is a **clinical consequence** of hyperkalemia, it is a **secondary effect** resulting from persistent depolarization. - The initial membrane depolarization eventually leads to **sodium channel inactivation**, causing muscle weakness. - This is not the immediate membrane effect but rather a downstream consequence. *Decreased cardiac excitability* - This is also a **clinical consequence** of severe or prolonged hyperkalemia, not the immediate membrane effect. - The initial depolarization eventually leads to **persistent sodium channel inactivation**, depressing myocardial excitability and conduction. - This manifests as **bradycardia, widened QRS complexes, and potentially asystole**.

Question 475: A 70-year-old male with a history of hypertension has elevated plasma renin levels. Which of the following physiological mechanisms is most likely responsible for his hypertension?

A. Increased aldosterone secretion (Correct Answer)
B. Decreased sympathetic activity
C. Increased natriuresis
D. Decreased angiotensin II levels

Explanation: ***Increased aldosterone secretion*** - Elevated plasma renin levels lead to increased production of **angiotensin II**, which stimulates the adrenal cortex to secrete **aldosterone**. - **Aldosterone** causes sodium and water retention by the kidneys, leading to increased blood volume and consequently **hypertension**. *Decreased sympathetic activity* - **Decreased sympathetic activity** would typically lead to a reduction in heart rate, contractility, and peripheral vasoconstriction, which would tend to **lower blood pressure**, not cause hypertension. - Furthermore, decreased sympathetic activity would generally lead to **decreased renin release**, which contradicts the patient's elevated renin levels. *Increased natriuresis* - **Natriuresis** is the excretion of sodium in the urine; an increase in natriuresis would lead to **decreased total body sodium** and thus a **reduction in blood pressure**. - Elevated renin levels would promote sodium retention (via aldosterone), thus suppressing natriuresis, which is counter to this option. *Decreased angiotensin II levels* - **Elevated renin levels** would lead to **increased production of angiotensin I** and subsequently **increased angiotensin II levels**, not decreased. - **Angiotensin II** is a potent vasoconstrictor and stimulates aldosterone release, both of which contribute to hypertension.

Question 476: A patient with severe dehydration presents with oliguria. What physiological mechanism is primarily responsible for the decreased urine output?

A. Increased glomerular filtration rate
B. Increased secretion of aldosterone
C. Increased reabsorption of sodium
D. Increased release of antidiuretic hormone (Correct Answer)

Explanation: ***Increased release of antidiuretic hormone*** - In **dehydration**, the body conserves water due to increased plasma osmolality and decreased blood volume, leading to the release of **antidiuretic hormone (ADH)** from the posterior pituitary. - ADH acts on the **collecting ducts** and **distal tubules** of the kidneys, increasing their permeability to water, thus promoting water reabsorption and reducing urine output (oliguria). *Increased glomerular filtration rate* - **Dehydration** typically leads to a **decreased glomerular filtration rate (GFR)** because reduced blood volume lowers renal blood flow and glomerular hydrostatic pressure. - A higher GFR would generally result in increased urine production, which contradicts the patient's symptom of **oliguria**. *Increased secretion of aldosterone* - While increased **aldosterone** secretion does occur in dehydration to conserve sodium and, subsequently, water, its primary role is to regulate **sodium reabsorption** in the distal tubules and collecting ducts, not directly reduce an existing large volume of filtered water. - Aldosterone's effect on water reabsorption is secondary to its effect on sodium, and it does not directly increase water permeability in the same way **ADH** does. *Increased reabsorption of sodium* - Increased **sodium reabsorption** is a compensatory mechanism in dehydration, driven primarily by **aldosterone**, to maintain extracellular fluid volume. - While water follows sodium reabsorption, the most direct and potent mechanism for reducing urine volume in the context of severe dehydration and oliguria is **ADH-mediated water reabsorption**.

Question 477: A patient with a recent diagnosis of diabetes insipidus is unable to concentrate urine properly. Which hormone is deficient in this condition?

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

Explanation: ***Antidiuretic hormone (ADH)*** - **Diabetes insipidus** is characterized by the kidneys' inability to conserve water, leading to excessive urination and thirst, due to a deficiency in **antidiuretic hormone (ADH)** or insensitivity to it. - **ADH** (also known as **vasopressin**) is crucial for **water reabsorption** in the renal tubules, allowing for proper urine concentration. *Aldosterone* - **Aldosterone** is a mineralocorticoid involved in **sodium reabsorption** and **potassium excretion**, primarily affecting blood pressure and electrolyte balance, not urine concentration in this manner. - Its deficiency or excess can lead to conditions like Addison's disease or Conn's syndrome, with different clinical presentations than diabetes insipidus. *Cortisol* - **Cortisol** is a glucocorticoid that plays a role in stress response, metabolism, and inflammation, but it does not directly regulate urine concentration. - Deficiencies often present as adrenal insufficiency with symptoms like fatigue, weight loss, and hypotension, distinct from diabetes insipidus. *Oxytocin* - **Oxytocin** is a hormone involved in uterine contractions during childbirth and milk ejection during lactation. - It has no role in the regulation of kidney water reabsorption or urine concentration.

Question 478: A patient with chronic hypertension is found to have elevated aldosterone levels. What is the impact of elevated aldosterone on renal sodium reabsorption and potassium excretion?

A. Increased sodium reabsorption and increased potassium excretion (Correct Answer)
B. Increased sodium reabsorption and decreased potassium excretion
C. Decreased sodium reabsorption and decreased potassium excretion
D. Decreased sodium reabsorption and increased potassium excretion

Explanation: ***Increased sodium reabsorption and increased potassium excretion*** - **Aldosterone** acts on the **principal cells** in the **collecting ducts** of the kidneys, stimulating the reabsorption of **sodium** via **ENaC channels**. - This increased sodium reabsorption creates a negative electrical potential, facilitating the secretion of **potassium** into the tubular lumen via **ROMK channels**. *Increased sodium reabsorption and decreased potassium excretion* - While aldosterone does increase **sodium reabsorption**, it concurrently **increases potassium excretion**, not decreases it. - Decreased potassium excretion in the presence of elevated aldosterone would lead to **hyperkalemia**, which is not the typical response. *Decreased sodium reabsorption and decreased potassium excretion* - **Aldosterone's primary role** is to increase **sodium reabsorption** to maintain blood volume and blood pressure. - Therefore, decreased sodium reabsorption is contrary to the known physiological action of aldosterone. *Decreased sodium reabsorption and increased potassium excretion* - Although **increased potassium excretion** is an effect of aldosterone, **decreased sodium reabsorption** is not. - This combination mechanism would result in **hypovolemia** and **hyponatremia**, which are not typical features of elevated aldosterone.

Question 479: Which component of the nephron is primarily responsible for the reabsorption of water, electrolytes, and nutrients?

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

Explanation: ***Proximal convoluted tubule*** - The **proximal convoluted tubule (PCT)** is responsible for the reabsorption of approximately 65-70% of filtered water and solutes. - Its cells have a high density of **mitochondria** and a **brush border** of microvilli, which increase surface area and provide energy for active transport of **sodium, glucose, amino acids, and other nutrients**. *Loop of Henle* - The **Loop of Henle** primarily establishes an **osmotic gradient** in the renal medulla, essential for concentrating urine. - It reabsorbs a significant amount of **water in the descending limb** and **sodium and chloride in the ascending limb**, but not nutrients like glucose or amino acids. *Distal convoluted tubule* - The **distal convoluted tubule (DCT)** plays a more specialized role in **fine-tuning** electrolyte and water balance under hormonal control. - It reabsorbs a smaller percentage of filtered substances and is influenced by hormones like **aldosterone** (sodium reabsorption) and **parathyroid hormone** (calcium reabsorption). *Collecting duct* - The **collecting duct** is the final site for **water reabsorption**, regulated primarily by **antidiuretic hormone (ADH)**. - It also contributes to the regulation of **acid-base balance** and **potassium secretion**, but is not responsible for bulk reabsorption of nutrients.

Question 480: A 60-year-old female presents with symptoms of dehydration. Which part of the nephron is primarily responsible for the reabsorption of water to correct this condition?

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

Explanation: ***Collecting duct*** - The **collecting duct** is primarily responsible for the **fine-tuning of water reabsorption** under the influence of **antidiuretic hormone (ADH)**. In dehydration, ADH levels increase significantly, making the collecting ducts highly permeable to water, thus reclaiming a large volume of water from the filtrate. - This crucial reabsorption in the collecting ducts helps to conserve body water and concentrate urine in states of **dehydration**, maintaining **fluid balance** and preventing further water loss. *Proximal convoluted tubule* - The **proximal convoluted tubule (PCT)** reabsorbs about **60-70% of filtered water** and solutes *unconditionally*, meaning it does not adjust significantly in response to hydration status. - While it reabsorbs a large volume of water, its role is primarily in bulk reabsorption, not in the regulated adjustments needed to correct dehydration. *Loop of Henle* - The **Loop of Henle** primarily creates a **concentration gradient** in the renal medulla, which is essential for the collecting duct to reabsorb water. - While water freely leaves the **descending limb** due to the osmotic gradient, the **ascending limb** is impermeable to water, so the loop itself does not directly adjust water reabsorption in response to dehydration beyond creating the necessary gradient. *Distal convoluted tubule* - The **distal convoluted tubule (DCT)** reabsorbs a small, variable amount of water and solutes, mainly under the influence of **aldosterone** for sodium and potassium balance. - While it has some role in water reabsorption, it is less significant than the collecting duct in the large-scale adjustments required for correcting **dehydration**.

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

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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