Renal Physiology — MCQs
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Orexins are implicated in all except:
Which of the following can reduce the glomerular filtration rate (GFR)?
A 78-year-old man presents with nausea and vomiting. On examination, he appears dehydrated, his abdomen is soft, and the jugular venous pressure is not elevated. Laboratory tests reveal hypernatremia, and his calculated free water deficit is approximately 3 L. In which part of the normal kidney is most of the water reabsorbed?
Reabsorption of water is maximum in which part of the nephron?
A patient is given a drug that dilates the afferent arteriole and contracts the efferent arteriole, leaving total renal vascular resistance unchanged. Which set of changes is likely to occur in the following table? (I = increased, D = decreased, NC = no change)
Which of the following does not cause relaxation of the urinary bladder?
What is the reason for the high sodium content in the renal medulla?
K+ secretion from the loop of Henle is decreased by which of the following medications?
If Renal Blood Flow is 1250 ml/min, calculate Renal Plasma Flow.
What percentage of cardiac output is supplied to the kidneys?
Renal Physiology Indian Medical PG Practice Questions and MCQs
Question 61: Orexins are implicated in all except:
- A. Wakefulness
- B. Sexual behaviour (Correct Answer)
- C. Appetite
- D. Alzheimer's disease
Explanation: **Explanation:** Orexins (also known as hypocretins) are neuropeptides produced by a small group of neurons in the **lateral hypothalamus**. They play a pivotal role in regulating the sleep-wake cycle and energy homeostasis. **1. Why "Sexual Behaviour" is the correct answer:** While orexins influence various hypothalamic functions, there is currently no established or significant evidence linking them directly to the regulation of **sexual behaviour**. Sexual behavior is primarily governed by the preoptic area of the hypothalamus and sex steroids (androgens/estrogens). **2. Analysis of Incorrect Options:** * **Wakefulness (Option A):** Orexins are essential for maintaining prolonged wakefulness. They stimulate monoaminergic and cholinergic neurons in the brainstem and forebrain. A deficiency in orexin signaling leads to **Narcolepsy Type 1** (characterized by excessive daytime sleepiness and cataplexy). * **Appetite (Option C):** As the name suggests (*orexis* = appetite), these peptides are potent stimulators of food intake. They integrate peripheral metabolic signals (like ghrelin and leptin) to regulate energy expenditure and hunger. * **Alzheimer’s Disease (Option D):** Recent research indicates that orexin levels are linked to beta-amyloid dynamics. Chronic sleep deprivation (driven by orexin overactivity) is associated with increased amyloid-beta plaque deposition, making orexin antagonists a potential area of study for Alzheimer's management. **Clinical Pearls for NEET-PG:** * **Suvorexant:** A dual orexin receptor antagonist (DORA) used clinically for the treatment of insomnia. * **Narcolepsy Pathophysiology:** Caused by the autoimmune destruction of orexin-producing neurons in the lateral hypothalamus. * **CSF Findings:** Low levels of Orexin-A (Hypocretin-1) in the cerebrospinal fluid is a diagnostic marker for Narcolepsy Type 1.
Question 62: Which of the following can reduce the glomerular filtration rate (GFR)?
- A. Norepinephrine
- B. Epinephrine
- C. Endothelium-derived nitric oxide (Correct Answer)
- D. Endothelin
Explanation: **Explanation:** The Glomerular Filtration Rate (GFR) is primarily determined by the balance of hydrostatic and oncotic pressures across the glomerular capillary membrane. The most critical factor regulating GFR is the tone of the afferent and efferent arterioles. **Why the Correct Answer is Right:** **Endothelium-derived Nitric Oxide (NO)** is a potent **vasodilator**. It is produced by the vascular endothelium and acts to decrease renal vascular resistance in both afferent and efferent arterioles. By dilating these vessels, NO increases renal blood flow (RBF) and maintains or **increases GFR**. Therefore, it does *not* reduce GFR; rather, it prevents excessive vasoconstriction. *(Note: In the context of the question provided, NO is the "odd one out" because it is a vasodilator, whereas the other three are vasoconstrictors that typically reduce GFR.)* **Analysis of Incorrect Options:** * **Norepinephrine & Epinephrine (A & B):** These are potent vasoconstrictors released by the sympathetic nervous system and adrenal medulla. They constrict both afferent and efferent arterioles (via $\alpha_1$ receptors), significantly **reducing GFR** and RBF during stress or hemorrhage. * **Endothelin (D):** This is a powerful peptide vasoconstrictor released by damaged vascular endothelium. It causes profound renal vasoconstriction and **reduces GFR**. It is often elevated in conditions like toxemia of pregnancy or acute renal failure. **High-Yield Clinical Pearls for NEET-PG:** * **Prostaglandins ($PGE_2$ and $PGI_2$):** Like NO, these are vasodilators that protect GFR. **NSAIDs** block these, leading to afferent vasoconstriction and potential acute kidney injury. * **Angiotensin II:** Preferentially constricts the **efferent arteriole** at low concentrations to maintain GFR when renal perfusion pressure is low. * **Atrial Natriuretic Peptide (ANP):** Increases GFR by dilating the afferent arteriole and constricting the efferent arteriole.
Question 63: A 78-year-old man presents with nausea and vomiting. On examination, he appears dehydrated, his abdomen is soft, and the jugular venous pressure is not elevated. Laboratory tests reveal hypernatremia, and his calculated free water deficit is approximately 3 L. In which part of the normal kidney is most of the water reabsorbed?
- A. Collecting ducts
- B. Proximal tubule (Correct Answer)
- C. Distal tubule
- D. Ascending loop of Henle
Explanation: **Explanation:** The correct answer is **B. Proximal tubule**. In a normal kidney, approximately **65-70% of the filtered water** is reabsorbed in the **Proximal Convoluted Tubule (PCT)**. This process is known as **obligatory water reabsorption**. It occurs isosmotically, meaning water follows the active reabsorption of solutes (primarily sodium via the Na+/K+ ATPase pump) through aquaporin-1 channels and paracellular pathways to maintain osmotic equilibrium. **Why other options are incorrect:** * **Collecting ducts:** While this is the site of **facultative water reabsorption** regulated by Antidiuretic Hormone (ADH), it only accounts for about 5-10% of total water reabsorption. It is crucial for final urine concentration but not for the bulk of volume recovery. * **Distal tubule:** The early distal tubule is part of the "diluting segment" and is relatively impermeable to water. * **Ascending loop of Henle:** This segment (specifically the Thick Ascending Limb) is **impermeable to water**. It actively reabsorbs solutes (Na+/K+/2Cl-) without water, which is essential for creating the medullary osmotic gradient. **High-Yield Clinical Pearls for NEET-PG:** * **PCT:** Always reabsorbs a constant fraction of filtered load (Glomerulotubular balance). * **Descending Loop of Henle:** Highly permeable to water but impermeable to solutes (concentrating segment). * **Countercurrent Multiplier:** Established by the Loop of Henle; **Countercurrent Exchanger** is the VASA RECTA. * **Clinical Correlation:** In dehydration (as seen in this patient), ADH levels rise to increase water permeability in the collecting ducts, but the PCT still performs the "heavy lifting" of bulk reabsorption.
Question 64: Reabsorption of water is maximum in which part of the nephron?
- A. Proximal Convoluted Tubule (PCT) (Correct Answer)
- B. Distal Convoluted Tubule (DCT)
- C. Collecting duct
- D. Loop of Henle
Explanation: **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for the reabsorption of water and solutes. Approximately **65-70%** of the filtered water is reabsorbed here. This process is known as **obligatory water reabsorption**, as it occurs regardless of the body's hydration status. It is driven by the active transport of sodium (Na+); as sodium is pumped out of the tubule, water follows passively via osmosis through **Aquaporin-1** channels to maintain osmotic equilibrium. **Analysis of Incorrect Options:** * **Loop of Henle:** While the descending limb is permeable to water, the entire loop accounts for only about **15%** of water reabsorption. The ascending limb is virtually impermeable to water. * **Distal Convoluted Tubule (DCT):** This segment is relatively impermeable to water and is primarily involved in the fine-tuning of electrolytes. * **Collecting Duct:** This is the site of **facultative water reabsorption** (about 5-10%), regulated by **Antidiuretic Hormone (ADH)** acting on **Aquaporin-2** channels. While crucial for concentrating urine during dehydration, the absolute volume reabsorbed here is much lower than in the PCT. **High-Yield NEET-PG Pearls:** * **Isotonic Reabsorption:** Fluid leaving the PCT is always **isotonic** to plasma (300 mOsm/L) because water and solutes are reabsorbed in equal proportions. * **Glucose & Amino Acids:** 100% of filtered glucose and amino acids are reabsorbed in the PCT (via SGLT-2 and SGLT-1). * **Carbonic Anhydrase:** The PCT is the major site for bicarbonate reabsorption, making it the target for the diuretic Acetazolamide.
Question 65: A patient is given a drug that dilates the afferent arteriole and contracts the efferent arteriole, leaving total renal vascular resistance unchanged. Which set of changes is likely to occur in the following table? (I = increased, D = decreased, NC = no change)
- A. Table option A
- B. Table option B
- C. Table option C (Correct Answer)
- D. Table option D
Explanation: ***Table option C*** - **Afferent dilation** and **efferent constriction** increases **glomerular capillary pressure**, leading to **increased GFR** while maintaining constant **RPF** since total **renal vascular resistance** remains unchanged. - **Filtration fraction** (FF = GFR/RPF) increases because GFR rises while RPF stays constant, and **glomerular capillary pressure** increases due to enhanced blood flow into and restricted flow out of glomerular capillaries. *Table option A* - This option likely shows **decreased GFR**, which contradicts the expected effect of increased **glomerular capillary pressure** from the described vascular changes. - The combination of **afferent dilation** and **efferent constriction** should enhance **filtration pressure**, not reduce it. *Table option B* - This option probably indicates **decreased filtration fraction**, which is incorrect since GFR increases more than any potential RPF changes. - **Glomerular capillary pressure** changes are likely incorrectly predicted, not accounting for the **Starling forces** modification from vascular resistance alterations. *Table option D* - This option may show **decreased glomerular capillary pressure**, which contradicts the hemodynamic effects of **afferent vasodilation** and **efferent vasoconstriction**. - The **RPF changes** are likely incorrectly predicted, not considering that total **renal vascular resistance** compensation maintains overall renal blood flow.
Question 66: Which of the following does not cause relaxation of the urinary bladder?
- A. Sympathetic Stimulation
- B. Muscarinic receptors (Correct Answer)
- C. Beta receptors
- D. None of the above
Explanation: To understand the physiology of micturition, one must distinguish between the actions of the sympathetic and parasympathetic nervous systems on the detrusor muscle and the internal sphincter. ### **Explanation** The **detrusor muscle** (the main muscle of the bladder wall) is responsible for bladder contraction during voiding. * **Muscarinic Receptors (M3):** These are the primary receptors of the **parasympathetic** nervous system (via the Pelvic nerve). Stimulation of M3 receptors leads to **contraction** of the detrusor muscle to facilitate emptying. Therefore, muscarinic stimulation causes contraction, not relaxation. ### **Analysis of Other Options** * **Sympathetic Stimulation:** The sympathetic nervous system (via the Hypogastric nerve) promotes **bladder filling** (continence). It achieves this by relaxing the detrusor muscle and contracting the internal urethral sphincter. * **Beta Receptors ($\beta_3$):** These are the specific sympathetic receptors located in the detrusor muscle. Activation of $\beta_3$ receptors leads to **relaxation** of the bladder wall, allowing it to expand and store urine. ### **NEET-PG High-Yield Pearls** 1. **Mnemonic for Bladder Control:** * **P**arasympathetic = **P**ee (Contracts detrusor, relaxes sphincter). * **S**ympathetic = **S**tore (Relaxes detrusor via $\beta_3$, contracts internal sphincter via $\alpha_1$). 2. **Clinical Correlation:** **Mirabegron** is a $\beta_3$ agonist used to treat overactive bladder by promoting detrusor relaxation. 3. **Anticholinergics:** Drugs like Oxybutynin block muscarinic receptors to prevent involuntary bladder contractions in urge incontinence. 4. **Internal vs. External Sphincter:** The internal sphincter is involuntary (autonomic), while the external sphincter is voluntary (Somatic - Pudendal nerve).
Question 67: What is the reason for the high sodium content in the renal medulla?
- A. The loop of Henle employs a countercurrent mechanism. (Correct Answer)
- B. There is increased blood flow to the vasa recta.
- C. The proximal convoluted tubule exhibits increased Na+ excretion.
- D. The proximal convoluted tubule demonstrates increased Na+ absorption.
Explanation: ### Explanation The high osmolarity (and high sodium content) of the renal medulla is essential for the kidney's ability to concentrate urine. This is achieved through the **Countercurrent Mechanism**, which consists of two components: the **Countercurrent Multiplier** (Loop of Henle) and the **Countercurrent Exchanger** (Vasa Recta). **1. Why Option A is Correct:** The **Loop of Henle** acts as a countercurrent multiplier. The thick ascending limb actively pumps Na+, K+, and Cl- into the medullary interstitium via the NKCC2 transporter but remains impermeable to water. This deposition of solutes creates a hypertonic medullary environment. The countercurrent flow (descending vs. ascending) allows this effect to be "multiplied," building a steep osmotic gradient from the cortex (300 mOsm/L) to the deep medulla (1200 mOsm/L). **2. Why the Other Options are Incorrect:** * **Option B:** Increased blood flow to the vasa recta would actually **"wash out"** the medullary gradient. For the gradient to be maintained, blood flow in the vasa recta must be slow and sluggish. * **Option C:** Increased Na+ excretion in the PCT would lead to natriuresis and would not contribute to the specific interstitial gradient of the medulla. * **Option D:** While the PCT reabsorbs ~65% of filtered sodium, this reabsorption is **isosmotic** (water follows sodium). Therefore, it does not contribute to the hypertonicity of the medulla; it merely reduces the total volume of the filtrate. **High-Yield Pearls for NEET-PG:** * **NKCC2 Transporter:** Target of Loop Diuretics (Furosemide); located in the Thick Ascending Limb. * **Urea Recycling:** Responsible for nearly 50% of the medullary hyperosmolarity; facilitated by ADH-dependent UT-A1 transporters in the collecting ducts. * **Vasa Recta:** Acts as an exchanger to maintain the gradient without consuming ATP, using a hairpin loop structure.
Question 68: K+ secretion from the loop of Henle is decreased by which of the following medications?
- A. Furosemide
- B. Thiazide
- C. Spironolactone (Correct Answer)
- D. Acetazolamide
Explanation: **Explanation:** The secretion of Potassium ($K^+$) primarily occurs in the **Principal cells** of the Late Distal Convoluted Tubule (DCT) and the Collecting Duct, rather than the Loop of Henle (note: while the question mentions the Loop of Henle, the physiological mechanism of $K^+$ secretion is the hallmark of the distal nephron). **Why Spironolactone is correct:** Spironolactone is a **Potassium-sparing diuretic** that acts as a competitive antagonist to **Aldosterone** receptors in the principal cells. Aldosterone normally promotes $Na^+$ reabsorption and $K^+$ secretion by increasing the activity of apical $ENaC$ channels and $ROMK$ channels. By blocking these receptors, Spironolactone directly inhibits the secretion of $K^+$ into the tubular lumen, leading to decreased $K^+$ excretion. **Why the other options are incorrect:** * **Furosemide (Loop Diuretic):** Inhibits the $Na^+-K^+-2Cl^-$ symporter in the Thick Ascending Loop of Henle. It **increases** $K^+$ secretion downstream due to increased sodium delivery to the distal tubule and volume depletion-induced hyperaldosteronism. * **Thiazides:** Inhibit the $Na^+-Cl^-$ symporter in the early DCT. Like loop diuretics, they cause **hypokalemia** by increasing distal $Na^+$ delivery, which stimulates $K^+$ secretion. * **Acetazolamide (CA Inhibitor):** Acts on the proximal tubule. It increases $K^+$ secretion by increasing distal delivery of $Na^+$ and bicarbonate, which acts as a non-reabsorbable anion, pulling $K^+$ into the lumen. **High-Yield Clinical Pearls for NEET-PG:** * **Site of $K^+$ Regulation:** While 65% of $K^+$ is reabsorbed in the PCT and 27% in the Loop of Henle, the **Late DCT and Collecting Duct** are the primary sites for regulated $K^+$ secretion. * **Aldosterone Paradox:** Aldosterone is the most important physiological regulator of $K^+$ secretion. * **Side Effect:** Spironolactone can cause **hyperkalemia** and **gynecomastia** (due to its non-specific anti-androgenic effects). Eplerenone is a more selective alternative.
Question 69: If Renal Blood Flow is 1250 ml/min, calculate Renal Plasma Flow.
- A. 690 ml/min (Correct Answer)
- B. 650 ml/min
- C. 710 ml/min
- D. 670 ml/min
Explanation: ### Explanation **1. Understanding the Concept** Renal Plasma Flow (RPF) is the volume of plasma that reaches the kidneys per unit time. Blood consists of two main components: cellular elements (primarily Red Blood Cells) and plasma. The proportion of blood occupied by RBCs is the **Hematocrit (Hct)**. To calculate RPF from Renal Blood Flow (RBF), we use the formula: **RPF = RBF × (1 – Hematocrit)** In standard physiological calculations (unless specified otherwise), the average human hematocrit is taken as **45% (0.45)**. * Calculation: $1250 \times (1 - 0.45)$ * $1250 \times 0.55 = \mathbf{687.5 \text{ ml/min}}$ * Rounding to the nearest option gives **690 ml/min**. **2. Analysis of Options** * **Option A (690 ml/min):** Correct. This aligns with the standard calculation using a normal hematocrit of 45%. * **Option B (650 ml/min):** Incorrect. This would imply a hematocrit of 48%, which is higher than the standard physiological average used for such questions. * **Option C (710 ml/min):** Incorrect. This would imply a hematocrit of approximately 43%. * **Option D (670 ml/min):** Incorrect. This would imply a hematocrit of approximately 46.4%. **3. High-Yield Facts for NEET-PG** * **Filtration Fraction (FF):** The ratio of GFR to RPF ($FF = GFR / RPF$). Normal value is ~20% (0.2). * **Gold Standard for RPF:** Para-aminohippuric acid (PAH) clearance is used to estimate Effective Renal Plasma Flow (ERPF) because it is both filtered and secreted. * **RBF Distribution:** Although the kidneys account for only 0.5% of body weight, they receive ~20-25% of the Cardiac Output. * **Autoregulation:** RBF and GFR remain constant between a Mean Arterial Pressure (MAP) of 80–180 mmHg due to the myogenic mechanism and tubuloglomerular feedback.
Question 70: What percentage of cardiac output is supplied to the kidneys?
- A. 10%
- B. 20% (Correct Answer)
- C. 30%
- D. None of the above
Explanation: **Explanation** The kidneys are highly vascular organs that receive a disproportionately large share of the cardiac output (CO) relative to their weight. In a healthy adult, the total renal blood flow (RBF) is approximately **1100–1200 mL/min**, which constitutes roughly **20% to 25%** of the total cardiac output (assuming a CO of 5–6 L/min). **Why 20% is Correct:** This high blood flow is not required to meet the metabolic (nutritional) demands of the renal tissue itself, but rather to ensure a high **Glomerular Filtration Rate (GFR)**. This allows the kidneys to precisely regulate volume status, electrolyte balance, and the excretion of waste products by processing the entire plasma volume multiple times a day. **Analysis of Incorrect Options:** * **10% (Option A):** This is too low. While the kidneys represent only about 0.5% of total body weight, they require significantly more than 10% of CO to maintain effective filtration pressures. * **30% (Option B):** This is too high for resting physiology. While RBF can fluctuate, 30% exceeds the standard physiological range for a healthy individual at rest. **High-Yield NEET-PG Pearls:** * **Oxygen Consumption:** Despite receiving 20% of CO, the kidneys only consume about 7-10% of the body's total oxygen. Most of this oxygen is used for the active reabsorption of sodium in the proximal tubules. * **Regional Distribution:** Blood flow is not uniform; the **Renal Cortex** receives the vast majority (~90%), while the **Medulla** receives very little (~1-2% via the vasa recta). This low medullary flow is essential for maintaining the osmotic gradient required for urine concentration. * **Autoregulation:** Renal blood flow is kept constant between a Mean Arterial Pressure (MAP) of **80 to 180 mmHg** via the myogenic mechanism and tubuloglomerular feedback.
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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