Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 21: A 50-year-old female goes into shock with a blood pressure of 50 mmHg. Which of the following concerning her Glomerular Filtration Rate (GFR) is most likely?

A. Myogenic autoregulation will keep GFR constant.
B. Angiotensin II release will maintain normal GFR.
C. The macula densa will cause constriction of the afferent arteriole to maintain GFR.
D. GFR will virtually cease. (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The Glomerular Filtration Rate (GFR) is primarily driven by the **Glomerular Hydrostatic Pressure ($P_{GC}$)**. Under normal physiological conditions, the kidneys maintain a constant GFR through **autoregulation**, which functions effectively only when the Mean Arterial Pressure (MAP) is between **80 mmHg and 180 mmHg**. In this patient, the blood pressure is **50 mmHg**, which falls significantly below the lower limit of the autoregulatory range. At this pressure, the hydrostatic pressure in the glomerular capillaries is insufficient to overcome the opposing forces (Colloid Osmotic Pressure and Bowman’s Space Hydrostatic Pressure). Consequently, the net filtration pressure drops to near zero, and **GFR virtually ceases**, leading to oliguria or anuria. **2. Why the Other Options are Wrong:** * **Option A:** Myogenic autoregulation (the reflex contraction of the afferent arteriole in response to stretch) is only effective within the **80–180 mmHg** range. Below 80 mmHg, the arteriole is already maximally dilated and cannot compensate further. * **Option B:** While Angiotensin II is released during shock to constrict the efferent arteriole (to preserve GFR), it cannot compensate for a massive drop in systemic pressure to 50 mmHg. The systemic hypotension is too severe for local hormonal control to maintain a "normal" GFR. * **Option C:** The Macula Densa mechanism (Tubuloglomerular Feedback) would actually cause **vasodilation** of the afferent arteriole in response to low NaCl delivery, not constriction, in an attempt to increase GFR. **3. High-Yield Clinical Pearls for NEET-PG:** * **Autoregulatory Range:** 80–180 mmHg (MAP). * **Primary Site of Autoregulation:** Afferent Arteriole. * **Formula:** $Net\ Filtration\ Pressure = (P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS})$. * **Clinical Correlation:** Severe hypotension leading to cessation of GFR is the precursor to **Prerenal Azotemia** and, if prolonged, **Acute Tubular Necrosis (ATN)**.

Question 22: Which substance is secreted by the proximal convoluted tubule (PCT)?

A. Sodium
B. Glucose
C. Hydrogen ions (Correct Answer)
D. Amino acids

Explanation: ### Explanation The **Proximal Convoluted Tubule (PCT)** is the most metabolically active segment of the nephron, responsible for both bulk reabsorption and selective secretion. **Why Hydrogen ions (H⁺) is correct:** The PCT plays a vital role in acid-base balance. It secretes **Hydrogen ions** into the tubular lumen, primarily via the **Sodium-Hydrogen Exchanger 3 (NHE3)**. This secretion is coupled with the reabsorption of filtered bicarbonate ($HCO_3^-$). For every $H^+$ ion secreted, one $HCO_3^-$ molecule is returned to the blood, making the PCT the primary site for maintaining the body's alkali reserve. **Why the other options are incorrect:** * **Sodium (A):** Sodium is **reabsorbed** (not secreted) in the PCT. Approximately 65% of filtered sodium is reabsorbed here through various symporters and antiporters. * **Glucose (B) & Amino acids (D):** These are essential nutrients that are **100% reabsorbed** in the early PCT under normal physiological conditions. Glucose is reabsorbed via **SGLT-2** (and some SGLT-1), while amino acids use specific sodium-coupled transporters. Their presence in urine (glycosuria/aminoaciduria) indicates a pathological state or exceeding the renal threshold. **High-Yield NEET-PG Pearls:** * **Secreted substances in PCT:** $H^+$, Ammonium ($NH_4^+$), Creatinine, and organic acids/bases (e.g., Penicillin, PAH, Urate). * **Carbonic Anhydrase:** The enzyme essential for $H^+$ secretion and $HCO_3^-$ reabsorption in the PCT. **Acetazolamide** inhibits this enzyme, leading to proximal renal tubular acidosis. * **Fanconi Syndrome:** A clinical condition resulting from generalized dysfunction of the PCT, leading to the wasting of glucose, amino acids, phosphate, and bicarbonate in the urine.

Question 23: Which part of the nephron is the dilution segment?

A. Ascending limb of the Loop of Henle (Correct Answer)
B. Descending limb of the Loop of Henle
C. Collecting tubule
D. Proximal tubule

Explanation: ### Explanation The correct answer is **A. Ascending limb of the Loop of Henle**. **Why it is the Diluting Segment:** The Thick Ascending Limb (TAL) of the Loop of Henle is termed the "diluting segment" because it is **impermeable to water** but actively reabsorbs solutes (Na⁺, K⁺, and Cl⁻) via the **NKCC2 transporter**. As electrolytes are pumped out into the medullary interstitium while water remains trapped in the tubule, the tubular fluid becomes increasingly dilute (hypotonic) relative to plasma. By the time the fluid reaches the distal convoluted tubule, its osmolarity drops to approximately 100 mOsm/L. **Analysis of Incorrect Options:** * **B. Descending limb of the Loop of Henle:** This is the "concentrating segment." It is highly permeable to water but impermeable to solutes. Water leaves the tubule via osmosis, making the tubular fluid hypertonic. * **C. Collecting tubule:** While dilution can occur here in the absence of ADH, it is primarily responsible for the final concentration of urine under the influence of ADH (Vasopressin). * **D. Proximal tubule:** Reabsorption here is **isostatic**. Both water and solutes are reabsorbed in equal proportions, so the tubular fluid remains isotonic (300 mOsm/L) to plasma. **NEET-PG High-Yield Pearls:** * **NKCC2 Transporter:** This is the target of **Loop Diuretics** (e.g., Furosemide). By inhibiting this transporter, these drugs abolish the corticomedullary gradient, preventing both dilution and concentration of urine. * **Bartter Syndrome:** A genetic defect in the NKCC2 transporter or associated channels in the TAL, mimicking chronic loop diuretic use. * **Countercurrent Multiplier:** The TAL provides the "single effect" that drives the countercurrent multiplier system, essential for the kidney's ability to concentrate urine.

Question 24: In the kidney, which of the following statements is true?

A. The osmolarity of tubular fluid in the PCT gradually increases.
B. The osmolarity of fluid in the DCT is more than in Bowman's capsule.
C. Fluid coming out of the descending limb of the loop of Henle is hypotonic.
D. The osmolarity of the medullary interstitium is greater than that of plasma. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** The core function of the renal medulla is to maintain a **hypertonic medullary interstitium** to facilitate water reabsorption. While plasma osmolarity is approximately **300 mOsm/L**, the medullary interstitium increases progressively from the cortex-medulla junction down to the papilla, reaching up to **1200–1400 mOsm/L**. This gradient is established by the **Countercurrent Multiplier** (Loop of Henle) and maintained by the **Countercurrent Exchanger** (Vasa Recta) and urea recycling. **2. Why the Other Options are Incorrect:** * **Option A:** In the Proximal Convoluted Tubule (PCT), water and solutes are reabsorbed in equal proportions (**Iso-osmotic reabsorption**). Therefore, the tubular fluid remains **isotonic** (300 mOsm/L) throughout the PCT. * **Option B:** The Distal Convoluted Tubule (DCT) receives fluid from the Thick Ascending Limb (TAL), which is the "diluting segment." Because solutes are removed without water, the fluid entering the DCT is **hypotonic** (~100–150 mOsm/L), making it less concentrated than the fluid in Bowman’s capsule (300 mOsm/L). * **Option C:** The Descending Limb of the Loop of Henle is highly permeable to water but not to solutes. As it descends into the hypertonic medulla, water leaves the tubule, making the fluid **hypertonic** (reaching 1200 mOsm/L at the bend). **3. High-Yield Clinical Pearls for NEET-PG:** * **Obligatory Reabsorption:** 65% of water is reabsorbed in the PCT regardless of ADH status. * **Diluting Segment:** The Thick Ascending Limb (TAL) is impermeable to water; this is the site of action for **Loop Diuretics** (Furosemide), which inhibit the Na⁺-K⁺-2Cl⁻ cotransporter. * **ADH Action:** ADH acts on the late DCT and Collecting Ducts via **Aquaporin-2** channels to concentrate urine. * **Urea:** Responsible for nearly 50% of the hypertonicity of the renal medullary interstitium.

Question 25: Destruction of the zona glomerulosa will deplete which hormone?

A. Aldosterone (Correct Answer)
B. Cortisol
C. Testosterone
D. Catecholamines

Explanation: **Explanation:** The adrenal gland is divided into an outer cortex and an inner medulla. The adrenal cortex consists of three distinct histological layers, often remembered by the mnemonic **"GFR"** (from superficial to deep), which correspond to the hormones they produce: **"Salt, Sugar, Sex."** 1. **Zona Glomerulosa (Outer):** Produces Mineralocorticoids, primarily **Aldosterone**. It is regulated by Angiotensin II and extracellular potassium levels. Destruction of this layer directly leads to a depletion of aldosterone, resulting in hyperkalemia and hyponatremia. 2. **Zona Fasciculata (Middle):** Produces Glucocorticoids, primarily **Cortisol**. It is the largest layer and is regulated by ACTH. 3. **Zona Reticularis (Inner):** Produces Androgens, such as Dehydroepiandrosterone (DHEA) and **Testosterone** precursors. **Analysis of Incorrect Options:** * **B. Cortisol:** Produced by the Zona Fasciculata, not the Glomerulosa. * **C. Testosterone:** Primarily produced by the testes in males; adrenal contributions come from the Zona Reticularis. * **D. Catecholamines:** These (Epinephrine and Norepinephrine) are produced by the **Adrenal Medulla** (chromaffin cells), which is embryologically derived from the neural crest. **NEET-PG High-Yield Pearls:** * **Conn’s Syndrome:** Primary hyperaldosteronism usually caused by an adenoma in the zona glomerulosa. * **Addison’s Disease:** Primary adrenal insufficiency involving destruction of all three cortical layers, leading to deficiencies in aldosterone, cortisol, and androgens. * **Regulation:** Unlike the Fasciculata and Reticularis, the Zona Glomerulosa is relatively independent of the anterior pituitary (ACTH) and is primarily controlled by the **Renin-Angiotensin-Aldosterone System (RAAS)**.

Question 26: Anuria is defined as urine output less than:

A. 4 ml/hr (Correct Answer)
B. 8 ml/hr
C. 12 ml/hr
D. 16 ml/hr

Explanation: **Explanation:** In clinical physiology, urine output is a critical indicator of renal perfusion and function. The definitions of reduced urine output are categorized based on the volume produced over a 24-hour period or per hour. **1. Why Option A is Correct:** **Anuria** is clinically defined as a urine output of **less than 100 ml in 24 hours**. To find the hourly rate, we divide 100 ml by 24 hours, which equals approximately **4.16 ml/hr**. Therefore, 4 ml/hr is the standard threshold used to define anuric states in a clinical setting. Anuria usually indicates a serious condition such as complete urinary tract obstruction, bilateral renal artery occlusion, or severe acute tubular necrosis. **2. Why Other Options are Incorrect:** * **Options B, C, and D (8, 12, and 16 ml/hr):** These values fall within the range of **Oliguria**. Oliguria is defined as urine output between **100 ml and 400 ml per day**. * 400 ml / 24 hours ≈ **17 ml/hr**. * Any value between 4 ml/hr and 17 ml/hr signifies oliguria rather than anuria. **3. High-Yield Clinical Pearls for NEET-PG:** * **Oliguria:** <400 ml/day (or <0.5 ml/kg/hr in adults). This is the minimum volume required to excrete the daily solute load (approx. 600 mOsm). * **Polyuria:** >3 Liters/day (commonly seen in Diabetes Mellitus and Diabetes Insipidus). * **Fixed Specific Gravity (1.010):** Known as **Isosthenuria**, this indicates chronic renal failure where the kidney loses its ability to concentrate or dilute urine. * **Urine Output Monitoring:** It is the most sensitive non-invasive indicator of cardiac output and tissue perfusion in critically ill patients.

Question 27: What is the approximate length of the distal convoluted tubule?

A. 5 mm (Correct Answer)
B. 2 mm
C. 12 mm
D. 8 mm

Explanation: **Explanation:** The **Distal Convoluted Tubule (DCT)** is a critical segment of the nephron responsible for the fine-tuning of electrolytes and acid-base balance. Anatomically, it begins at the macula densa (at the end of the thick ascending limb) and extends to the collecting duct. **1. Why Option A (5 mm) is Correct:** In standard physiological texts (such as Ganong and Guyton), the DCT is described as being approximately **5 mm** in length. It is significantly shorter than the Proximal Convoluted Tubule (PCT), which measures about 15 mm. Despite its shorter length, the DCT plays a vital role in the reabsorption of sodium and calcium (regulated by PTH) and is the primary site of action for thiazide diuretics. **2. Analysis of Incorrect Options:** * **Option B (2 mm):** This is too short for the DCT. This length is more characteristic of specific sub-segments or transition zones within the renal medulla. * **Option C (12 mm):** This value is closer to the length of the **Proximal Convoluted Tubule (PCT)**, which is the longest and most convoluted part of the nephron (approx. 12–15 mm). * **Option D (8 mm):** While closer than other options, 8 mm overestimates the average length of the DCT in a standard human nephron. **High-Yield Clinical Pearls for NEET-PG:** * **Histology:** Unlike the PCT, the DCT **lacks a brush border** (microvilli), making its lumen appear clearer and more distinct under a microscope. * **Macula Densa:** The very beginning of the DCT contains specialized cells called the macula densa, which sense distal sodium chloride delivery and regulate the **Tubuloglomerular Feedback (TGF)**. * **Hormonal Control:** The late DCT and collecting duct are the sites where **Aldosterone** acts to increase sodium reabsorption and potassium secretion. * **Diuretic Site:** Thiazides inhibit the **Na+-Cl- symporter** specifically in the early DCT.

Question 28: Sympathectomy of the kidney results in which of the following responses?

A. No change to the glomerular filtration rate, nor to the urine output
B. Decreased urine output
C. Reduction of glomerular filtration rate and decreased urinary output
D. Increased urine output (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Increased urine output.** **Mechanism:** The kidneys are primarily innervated by the sympathetic nervous system (SNS). Under normal physiological conditions, sympathetic tone causes tonic vasoconstriction of the renal arterioles (both afferent and efferent) and stimulates the release of renin from juxtaglomerular cells. When a **sympathectomy** (denervation) is performed, the sympathetic vasoconstrictor tone is abolished. This leads to: 1. **Vasodilation:** Primarily of the afferent arterioles, which increases renal blood flow (RBF). 2. **Pressure Diuresis/Natriuresis:** The loss of sympathetic input reduces proximal tubular sodium and water reabsorption. 3. **Decreased Renin:** Reduced SNS activity lowers Renin-Angiotensin-Aldosterone System (RAAS) activation, further promoting the excretion of water and sodium. The net result of these changes is a significant increase in urine volume, often referred to as "denervation diuresis." **Analysis of Incorrect Options:** * **Option A:** Incorrect because the loss of sympathetic tone significantly alters renal hemodynamics and tubular handling of solutes, leading to measurable changes. * **Options B & C:** Incorrect because these describe the effects of *increased* sympathetic activity (e.g., during hemorrhage or stress), which causes vasoconstriction, reduced GFR, and fluid retention to maintain blood pressure. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Autoregulation:** Despite denervation, the kidney can still maintain a relatively constant GFR and RBF between mean arterial pressures of 80–180 mmHg via intrinsic mechanisms (Myogenic and Tubuloglomerular feedback). * **Transplanted Kidney:** A transplanted kidney is a "denervated" kidney. It initially exhibits diuresis and lacks the sympathetic reflex to decrease GFR during exercise or stress. * **Sympathetic Effect on Tubules:** SNS directly stimulates $\alpha_1$-receptors on renal tubular cells to increase $Na^+$ reabsorption. Loss of this signal (sympathectomy) leads to natriuresis.

Question 29: What is the most important extracellular buffer?

A. Bicarbonate (Correct Answer)
B. Plasma protein
C. Phosphate
D. Nitrate

Explanation: **Explanation:** The **Bicarbonate buffer system ($HCO_3^- / CO_2$)** is the most important extracellular fluid (ECF) buffer. Its primary importance stems not from its concentration or pKa, but from its status as an **"open system."** The lungs can rapidly regulate the concentration of $CO_2$ (the acid component), while the kidneys regulate the concentration of $HCO_3^-$ (the base component). This dual physiological control allows the body to maintain a stable pH of 7.4 despite constant metabolic acid production. **Analysis of Options:** * **B. Plasma Proteins:** These are important intracellular and intravascular buffers (e.g., Albumin), but they are less significant than bicarbonate in the overall ECF because their concentration is lower and they cannot be regulated as rapidly as $CO_2$. * **C. Phosphate:** While the phosphate buffer has a pKa (6.8) closer to physiological pH than bicarbonate, its concentration in the ECF is very low. It is, however, a **major intracellular buffer** and the most important **tubular buffer** in the kidneys (titratable acidity). * **D. Nitrate:** This is not a physiological buffer system in the human body. **NEET-PG High-Yield Pearls:** * **Most important ECF buffer:** Bicarbonate. * **Most important ICF buffer:** Proteins and Phosphates. * **Most important buffer in RBCs:** Hemoglobin (due to the imidazole group of Histidine). * **Henderson-Hasselbalch Equation:** $pH = pKa + \log ([Base]/[Acid])$. For the bicarbonate system, the ratio of $HCO_3^-$ to $CO_2$ is normally **20:1**.

Question 30: Which of the following are actions of angiotensin II?

A. Systemic vasoconstriction
B. Systemic vasodilation (Correct Answer)
C. Renal vasodilatation
D. Reabsorption of Na+ in the proximal renal tubule

Explanation: **Explanation** Angiotensin II (AT-II) is a potent effector peptide of the Renin-Angiotensin-Aldosterone System (RAAS), primarily known for its role in increasing blood pressure and maintaining fluid balance. **Why the Correct Answer is Right:** The question asks for the actions of Angiotensin II. However, there appears to be a discrepancy in the provided key. **Physiologically, Angiotensin II is a potent systemic vasoconstrictor.** If the "Correct Answer" is marked as **Systemic vasodilation**, it is likely a "negative" question (e.g., "Which of the following is NOT an action...") or represents a rare pharmacological context (like AT2 receptor stimulation, which is not the primary physiological effect). In standard NEET-PG physiology, Angiotensin II **never** causes systemic vasodilation; it increases Total Peripheral Resistance (TPR) to raise blood pressure. **Analysis of Options:** * **A. Systemic vasoconstriction:** This is the primary action of AT-II via **AT1 receptors** on vascular smooth muscle. * **C. Renal vasodilation:** Incorrect. AT-II causes **vasoconstriction** of both afferent and (preferentially) efferent arterioles to maintain GFR during low-pressure states. * **D. Reabsorption of Na+ in the proximal tubule:** This is a major direct action. AT-II stimulates the **Na+-H+ exchanger (NHE3)** in the PCT, increasing sodium and water reabsorption. **NEET-PG High-Yield Pearls:** 1. **Preferential Action:** AT-II constricts the **efferent arteriole** more than the afferent, which increases Filtration Fraction (FF). 2. **Adrenal Effect:** It stimulates the Zona Glomerulosa to release **Aldosterone**, leading to late distal tubule Na+ reabsorption and K+ secretion. 3. **Thirst:** It acts on the **Subfornical Organ (SFO)** in the brain to stimulate the thirst center. 4. **Receptors:** Most known effects (vasoconstriction, salt retention) are via **AT1 receptors**. AT2 receptors generally mediate vasodilation and anti-proliferation but are less dominant in adults.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

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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