Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 181: What is the normal inulin clearance in milliliters per minute?

A. 55 ml/min
B. 625 ml/min
C. 125 ml/min (Correct Answer)
D. 40 ml/min

Explanation: ### Explanation **Correct Answer: C. 125 ml/min** **The Underlying Concept:** Inulin is a fructose polymer that serves as the **gold standard** for measuring the **Glomerular Filtration Rate (GFR)**. This is because inulin is freely filtered by the glomeruli but is neither reabsorbed nor secreted by the renal tubules. Therefore, the amount of inulin filtered per unit time is exactly equal to the amount excreted in the urine. In a healthy adult male of average body surface area (1.73 m²), the normal GFR is approximately **125 ml/min**. **Analysis of Incorrect Options:** * **A. 55 ml/min:** This value is significantly lower than a healthy GFR and would indicate Stage 3 Chronic Kidney Disease (CKD). * **B. 625 ml/min:** This represents the normal **Effective Renal Plasma Flow (ERPF)**, typically measured using Para-aminohippuric acid (PAH) clearance. * **D. 40 ml/min:** This is a low value, often associated with the clearance of substances that undergo significant tubular reabsorption, such as urea (normal urea clearance is ~75 ml/min, but can drop lower depending on flow rate). **High-Yield Clinical Pearls for NEET-PG:** * **Creatinine Clearance:** In clinical practice, endogenous creatinine is used to estimate GFR. However, it slightly **overestimates** GFR (by ~10-20%) because a small amount of creatinine is secreted by the tubules. * **PAH Clearance:** Used to measure Renal Plasma Flow (RPF) because it is both filtered and almost completely secreted. * **Filtration Fraction (FF):** Calculated as GFR/RPF. Normal FF is approximately **20%** (125/625). * **Criteria for GFR Marker:** Must be non-toxic, not metabolized by the kidney, and freely filtered without tubular transport.

Question 182: Glomerular filtration rate (GFR) will be increased if:

A. There is increased hydrostatic pressure in Bowman's capsule
B. There is decreased oncotic pressure in glomerular capillaries (Correct Answer)
C. There is increased oncotic pressure in glomerular capillaries
D. There is decreased hydrostatic pressure in glomerular capillaries

Explanation: ### Explanation The Glomerular Filtration Rate (GFR) is determined by **Starling’s Forces**, which govern the movement of fluid across the glomerular membrane. The relationship is expressed by the formula: **GFR = $K_f$ × [(Pgc – Pbc) – (πgc – πbc)]** #### Why Option B is Correct **Decreased oncotic pressure in glomerular capillaries ($\pi_{gc}$)**: Plasma proteins (mainly albumin) exert oncotic pressure that "pulls" fluid back into the capillaries, opposing filtration. If this pressure decreases (e.g., in hypoalbuminemia or hemodilution), the net filtration pressure increases, leading to an **increase in GFR**. #### Why Other Options are Incorrect * **Option A (Increased hydrostatic pressure in Bowman's capsule):** This pressure ($P_{bc}$) opposes filtration. An increase (e.g., due to kidney stones or urinary tract obstruction) creates "back-pressure," which **decreases GFR**. * **Option C (Increased oncotic pressure in glomerular capillaries):** Higher protein concentration increases the "pull" of fluid into the blood, thereby **decreasing GFR**. * **Option D (Decreased hydrostatic pressure in glomerular capillaries):** Glomerular hydrostatic pressure ($P_{gc}$) is the primary driving force for filtration. A decrease (e.g., due to hypotension or afferent arteriole constriction) **decreases GFR**. --- ### High-Yield Clinical Pearls for NEET-PG * **The most important determinant of GFR** under physiological conditions is the **Glomerular Hydrostatic Pressure ($P_{gc}$)**. * **Afferent arteriole constriction** (via Adenosine/Sympathetics) decreases GFR; **Efferent arteriole constriction** (via Angiotensin II) increases GFR (up to a point). * **Hypoalbuminemia** (as seen in Nephrotic Syndrome or Liver Cirrhosis) leads to an initial increase in GFR due to decreased $\pi_{gc}$, though the overall pathology may eventually damage the filtration barrier. * **$K_f$ (Filtration Coefficient)** can be reduced by diseases that decrease surface area, such as Diabetes Mellitus or Chronic Hypertension.

Question 183: In a healthy individual on a normal diet, which of the following ions is completely reabsorbed in the renal tubules?

A. Na+
B. K+
C. Cl-
D. HCO3- (Correct Answer)

Explanation: **Explanation:** The correct answer is **HCO3- (Bicarbonate)**. In a healthy individual, the kidneys play a vital role in maintaining acid-base balance. Under normal physiological conditions, approximately **99.9% to 100%** of filtered bicarbonate is reabsorbed to maintain the alkaline reserve of the body. * **Mechanism:** About 80-85% of HCO3- is reabsorbed in the **Proximal Convoluted Tubule (PCT)** via the action of Carbonic Anhydrase, while the remainder is reabsorbed in the Thick Ascending Limb and the Intercalated cells of the distal nephron. **Why other options are incorrect:** * **Na+ and Cl-:** While these ions are reabsorbed in massive quantities (approx. 99%), they are never "completely" reabsorbed. A small fraction is always excreted in urine to maintain osmotic balance and volume status, depending on dietary intake. * **K+:** Potassium is unique because it undergoes both reabsorption (in PCT and Loop of Henle) and **secretion** (in the Distal Tubule and Collecting Duct). The kidneys regulate K+ levels primarily through controlled secretion; therefore, it is never completely reabsorbed. **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Threshold for HCO3-:** It is approximately **24-28 mEq/L**. If plasma levels exceed this, bicarbonate will appear in the urine. 2. **Carbonic Anhydrase Inhibitors (Acetazolamide):** These drugs act on the PCT to inhibit HCO3- reabsorption, leading to alkaline urine and metabolic acidosis. 3. **Glucose and Amino Acids:** Along with HCO3-, these are also substances that are 100% reabsorbed in the PCT under normal physiological limits (below their respective renal thresholds).

Question 184: In the kidneys, at which site does the 1,25 hydroxylation of Vitamin-D occur?

A. Collecting ducts
B. Glomerulus
C. Proximal convoluted tubules (Correct Answer)
D. Distal convoluted tubules

Explanation: **Explanation:** The activation of Vitamin D is a two-step hydroxylation process. The first step occurs in the liver (25-hydroxylation), and the second, final activation step occurs in the **Proximal Convoluted Tubule (PCT)** of the kidneys. **Why the PCT is correct:** The PCT cells contain the enzyme **1-alpha-hydroxylase**. This enzyme converts 25-hydroxycholecalciferol (Calcidiol) into 1,25-dihydroxycholecalciferol (**Calcitriol**), which is the biologically active form of Vitamin D. This process is tightly regulated by Parathyroid Hormone (PTH), which stimulates the enzyme in response to low serum calcium levels. **Why the other options are incorrect:** * **Collecting Ducts & Distal Convoluted Tubules:** While these segments are involved in electrolyte reabsorption and are targets for hormones like Aldosterone and ADH, they do not possess the enzymatic machinery (1-alpha-hydroxylase) required for Vitamin D activation. * **Glomerulus:** This is primarily a filtration barrier. It does not have a significant metabolic or endocrine role in steroid hormone activation. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The 1-alpha-hydroxylation in the PCT is the rate-limiting step of Vitamin D synthesis. * **Regulation:** 1-alpha-hydroxylase is **stimulated by PTH** and low phosphate, and **inhibited by FGF-23** and high levels of Calcitriol (negative feedback). * **Chronic Kidney Disease (CKD):** In CKD, the loss of PCT functional mass leads to a deficiency of 1-alpha-hydroxylase, resulting in hypocalcemia and secondary hyperparathyroidism (Renal Osteodystrophy). * **Sarcoidosis:** Macrophages in granulomas can also express 1-alpha-hydroxylase, leading to hypercalcemia.

Question 185: What is the minimal amount of urine required for waste product elimination in a normal person?

A. 100 ml
B. 500 ml (Correct Answer)
C. 100 ml
D. 2000 ml

Explanation: ### Explanation The correct answer is **500 ml** (Option B). This volume is clinically referred to as the **Obligatory Urine Volume**. **1. Underlying Medical Concept:** To maintain homeostasis, an average adult must excrete approximately **600 mOsm** of metabolic waste products (such as urea, creatinine, and uric acid) per day. The human kidney has a maximum concentrating ability of approximately **1200 mOsm/L**. To calculate the minimum volume required to flush out these solutes: * *Calculation:* $600\text{ mOsm/day} \div 1200\text{ mOsm/L} = 0.5\text{ L/day}$ (or **500 ml/day**). If a person excretes less than this amount, metabolic waste products will accumulate in the blood, leading to azotemia. **2. Analysis of Incorrect Options:** * **Options A & C (100 ml):** This volume is characteristic of **Anuria** (<100 ml/day). At this level, the kidneys cannot excrete the daily solute load, leading to rapid renal failure. * **Option D (2000 ml):** This represents a normal to high-normal daily urine output. While healthy, it is far above the *minimal* requirement for waste elimination. **3. Clinical Pearls for NEET-PG:** * **Oliguria:** Defined as urine output **<400 ml/day** in adults. This is the point where the kidneys can no longer maintain solute balance even with maximal concentration. * **Specific Gravity:** At the obligatory volume (500 ml), the urine specific gravity is typically at its maximum (~1.030). * **Isosthenuria:** The inability to concentrate or dilute urine (fixed at 300 mOsm/L), often seen in chronic renal failure. * **Water Diuresis:** The kidneys can dilute urine down to **50 mOsm/L**, allowing for a maximum urine output of up to 18–20 Liters/day.

Question 186: Tubuloglomerular feedback occurs in which structure?

A. Antidiuretic hormone secretion in the collecting duct
B. Constant solute load presented to the distal tubule (Correct Answer)
C. Macula densa
D. Balance between the ascending and descending limbs of the loop of Henle

Explanation: **Explanation:** **Tubuloglomerular Feedback (TGF)** is an intrinsic autoregulatory mechanism of the kidney designed to maintain a **constant solute load** (specifically NaCl) to the distal tubule, thereby stabilizing the Glomerular Filtration Rate (GFR) and protecting the distal nephron from fluid overload. 1. **Why Option B is Correct:** The primary physiological goal of TGF is to ensure that the delivery of solutes to the distal tubule remains relatively constant. When GFR increases, the flow rate and NaCl concentration at the Macula Densa increase. This triggers the Macula Densa to release signaling molecules (like Adenosine) that cause afferent arteriolar vasoconstriction, reducing GFR back to normal levels. This feedback loop ensures the distal tubule is not overwhelmed by excessive solute delivery. 2. **Why Other Options are Incorrect:** * **Option A:** ADH acts on the collecting duct to regulate water reabsorption (concentrating urine); it is not part of the TGF mechanism. * **Option C:** While the **Macula Densa** is the *sensor* for TGF, the question asks for the *structure/process* occurring (the feedback itself). In many competitive exams, the "constant solute load" is considered the functional definition of the feedback's purpose. * **Option D:** The balance between the limbs of the Loop of Henle refers to the Countercurrent Multiplier system, not TGF. **High-Yield NEET-PG Pearls:** * **Sensor:** Macula Densa (located in the thick ascending limb/early distal tubule). * **Effector:** Afferent Arteriole (Vasoconstriction via Adenosine/ATP). * **Juxtaglomerular Apparatus (JGA):** Comprises the Macula Densa, Lacis cells, and Juxtaglomerular (granular) cells. * **Clinical Correlation:** NSAIDs can interfere with renal autoregulation by inhibiting prostaglandins, which normally counteract excessive vasoconstriction, potentially leading to acute kidney injury (AKI).

Question 187: Which is the only inducible urinary buffer?

A. Bicarbonate
B. Phosphate
C. Ammonia (Correct Answer)
D. Plasma proteins

Explanation: **Explanation:** The correct answer is **Ammonia**. **Why Ammonia is the correct answer:** The kidneys maintain acid-base balance by excreting hydrogen ions ($H^+$). While phosphate is a major urinary buffer, its concentration is limited by dietary intake and filtration rates (fixed capacity). In contrast, **Ammonia ($NH_3$)** is the only **inducible buffer**. In states of chronic acidosis, the proximal convoluted tubule (PCT) cells upregulate the enzyme **glutaminase**, which metabolizes Glutamine into Ammonium ($NH_4^+$) and Bicarbonate. This adaptive response allows the kidney to significantly increase its acid-excretion capacity (up to 10-fold) during prolonged metabolic acidosis. **Why the other options are incorrect:** * **Bicarbonate:** It is not a urinary buffer for $H^+$ excretion; rather, it is the substance being reclaimed. In acidosis, bicarbonate is completely reabsorbed, not excreted to act as a buffer. * **Phosphate:** This is the primary "Titratable Acid" buffer. However, it is not inducible. Its amount in the urine depends on the filtered load and cannot be increased by the kidneys in response to acidosis. * **Plasma Proteins:** While proteins are excellent systemic buffers in the blood, they are not filtered by the glomerulus in significant amounts and thus do not serve as urinary buffers. **High-Yield NEET-PG Pearls:** * **Site of Production:** Ammonia is primarily produced in the **Proximal Tubule** from **Glutamine**. * **Diffusion Trapping:** $NH_3$ (gas) diffuses into the tubular lumen, reacts with $H^+$ to form $NH_4^+$ (ion), which is "trapped" and excreted because it cannot diffuse back. * **Titratable Acidity:** Refers to $H^+$ buffered by phosphate and other organic acids, but *excludes* ammonia. * **Chronic Acidosis:** The increase in $NH_4^+$ excretion is the most important renal compensatory mechanism for long-term acid-base disturbances.

Question 188: In high serum potassium levels, how does the kidney compensate?

A. Decreased reabsorption in the proximal convoluted tubule (PCT)
B. Increased secretion in the proximal convoluted tubule (PCT)
C. Increased secretion in the distal convoluted tubule (DCT) (Correct Answer)
D. Decreased reabsorption in the loop of Henle

Explanation: ### Explanation The kidney maintains potassium homeostasis primarily through regulated secretion in the distal nephron. While the majority of filtered potassium is reabsorbed in the proximal segments, the **Distal Convoluted Tubule (DCT)** and the **Cortical Collecting Duct (CCD)** are the sites where physiological adjustment occurs. **1. Why Option C is Correct:** In hyperkalemia (high serum potassium), the adrenal cortex is directly stimulated to release **Aldosterone**. Aldosterone acts on the **Principal cells** of the late DCT and collecting ducts to: * Increase the activity of **Na+/K+ ATPase** on the basolateral membrane, pumping K+ into the cell. * Increase the permeability of the luminal membrane to K+ by opening **ROMK (Renal Outer Medullary Potassium) channels**. This creates a steep concentration gradient, leading to **increased K+ secretion** into the tubular lumen to be excreted in urine. **2. Why Other Options are Incorrect:** * **Options A & B:** The PCT reabsorbs about 65% of filtered potassium, mostly via passive paracellular transport (solvent drag). This process is relatively constant and is not the primary site for active regulation or compensatory secretion in response to serum levels. * **Option D:** The Thick Ascending Limb of the Loop of Henle reabsorbs ~25-30% of K+ via the **NKCC2 transporter**. While loop diuretics can inhibit this, the body does not primarily "decrease reabsorption" here as a physiological compensation for hyperkalemia; it relies on distal secretion. **3. NEET-PG High-Yield Pearls:** * **Principal Cells:** Responsible for K+ secretion and Na+ reabsorption (site of Aldosterone action). * **Intercalated Cells (Type A):** Responsible for K+ **reabsorption** during hypokalemia via H+/K+ ATPase. * **Insulin & Beta-2 Agonists:** These shift K+ **into** cells (used in acute management of hyperkalemia). * **Acid-Base Link:** Acidosis generally causes hyperkalemia (H+ enters cells, K+ exits), while alkalosis causes hypokalemia.

Question 189: What is the filtration pressure in the glomeruli of the kidney?

A. 10 mm of Hg (Correct Answer)
B. 6 mm of Hg
C. 15 mm of Hg
D. 2.0 mm of Hg

Explanation: **Explanation:** The **Net Filtration Pressure (NFP)** is the total pressure that promotes filtration in the glomerulus. It is determined by the balance of Starling forces acting across the glomerular capillary membrane. The formula for NFP is: **NFP = Glomerular Hydrostatic Pressure (Pgc) – [Plasma Colloid Osmotic Pressure (πgc) + Bowman’s Capsule Hydrostatic Pressure (Pbc)]** Using standard physiological values: * **Pgc:** ~60 mm Hg (promotes filtration) * **πgc:** ~32 mm Hg (opposes filtration) * **Pbc:** ~18 mm Hg (opposes filtration) * **Calculation:** 60 – (32 + 18) = **10 mm Hg**. **Analysis of Options:** * **Option A (10 mm Hg):** This is the standard physiological value for NFP in a healthy adult, representing the effective pressure driving the formation of ultrafiltrate. * **Option B & C (6 and 15 mm Hg):** These values do not align with standard Starling force calculations. While NFP can fluctuate based on afferent/efferent arteriolar resistance, 10 mm Hg is the classic textbook value for NEET-PG. * **Option D (2.0 mm Hg):** This is too low to sustain an adequate Glomerular Filtration Rate (GFR) and would likely result in acute renal failure. **High-Yield Clinical Pearls for NEET-PG:** 1. **Glomerular Hydrostatic Pressure (60 mm Hg)** is the highest capillary hydrostatic pressure in the body, maintained by the high-resistance efferent arteriole. 2. **Autoregulation:** The kidney maintains a constant GFR and NFP between mean arterial pressures of **80–180 mm Hg** via the myogenic mechanism and tubuloglomerular feedback. 3. **Filtration Coefficient (Kf):** GFR = Kf × NFP. Diseases like diabetes mellitus reduce GFR by decreasing Kf (reducing surface area).

Question 190: Which change tends to increase Glomerular Filtration Rate (GFR)?

A. Increased afferent arteriolar resistance
B. Decreased efferent arteriolar resistance
C. Increased glomerular capillary filtration coefficient (Correct Answer)
D. Increased Bowman's capsule hydrostatic pressure

Explanation: ### Explanation The Glomerular Filtration Rate (GFR) is determined by the product of the **Glomerular Capillary Filtration Coefficient ($K_f$)** and the **Net Filtration Pressure (NFP)**. The relationship is expressed by the formula: **$GFR = K_f \times [(P_g - P_b) - (\pi_g - \pi_b)]$** #### Why Option C is Correct The **Filtration Coefficient ($K_f$)** is a measure of the product of the hydraulic conductivity and the surface area of the glomerular capillaries. An **increase in $K_f$** (e.g., through relaxation of glomerular mesangial cells) directly increases GFR. Conversely, diseases that reduce the number of functional capillaries or thicken the membrane (like chronic hypertension or diabetes) decrease $K_f$ and GFR. #### Why Other Options are Incorrect * **A. Increased afferent arteriolar resistance:** This reduces the blood flow into the glomerulus and decreases the glomerular hydrostatic pressure ($P_g$), thereby **decreasing GFR**. * **B. Decreased efferent arteriolar resistance:** This allows blood to leave the glomerulus more easily, lowering the $P_g$ and **decreasing GFR**. (Note: *Increasing* efferent resistance increases GFR, up to a certain point). * **D. Increased Bowman's capsule hydrostatic pressure ($P_b$):** This acts as a "back pressure" opposing filtration. Obstructions like kidney stones increase $P_b$, which **decreases GFR**. #### High-Yield Clinical Pearls for NEET-PG * **Mesangial Cells:** Contraction of these cells (stimulated by Angiotensin II or ADH) reduces surface area, decreasing $K_f$ and GFR. * **Autoregulation:** GFR is maintained between Mean Arterial Pressures of **75–160 mmHg** via the Myogenic mechanism and Tubuloglomerular feedback. * **Constriction Paradox:** While moderate efferent constriction increases GFR, *severe* constriction can decrease GFR because it significantly reduces renal plasma flow, leading to a rapid rise in colloid osmotic pressure.

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

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Sodium and Water Balance

Practice Questions

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

Practice Questions

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