Renal Physiology Practice Questions

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

Increased glomerular capillary filtration coefficient. ### 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.

Q: What is the maximum osmolarity of urine in the presence of ADH?

1200 mOsmol / L. ### Explanation **1. Why the Correct Answer is Right (C: 1200 mOsmol/L)** The maximum concentrating ability of the human kidney is approximately **1200 to 1400 mOsmol/L**. This process is governed by **Antidiuretic Hormone (ADH)**, also known as Vasopressin. * **Mechanism:** ADH acts on the V2 receptors of the late distal tubule and collecting ducts, inserting **Aquaporin-2 (AQP2)** channels. * **Countercurrent Multiplier:** The loop of Henle creates a hypertonic medullary interstitium (reaching 1200 mOsmol/L at the tip of the papilla). * **Equilibration:** In the presence of ADH, water is reabsorbed from the collecting duct into the hypertonic medulla until the tubular fluid reaches osmotic equilibrium with the surrounding interstitium. Thus, the final urine osmolarity matches the maximum medullary osmolarity. **2. Why the Other Options are Wrong** * **Option A (50 mOsmol/L):** This represents the **minimum** osmolarity of urine (most dilute) achieved in the absence of ADH (e.g., Diabetes Insipidus or excessive water intake). * **Option B (800 mOsmol/L):** While this is hypertonic compared to plasma (290 mOsmol/L), it does not represent the physiological maximum limit of the human kidney. * **Option D (500 mOsmol/L):** This is a moderately concentrated urine but is far below the maximum capacity of the medullary gradient. **3. NEET-PG High-Yield Pearls** * **Obligatory Urine Volume:** To excrete a daily solute load of 600 mOsmol, a human must produce at least **0.5 L/day** of urine ($600 \div 1200 = 0.5$). * **Urea Recycling:** Urea contributes nearly **50%** of the hypertonicity of the renal medullary interstitium. * **Vasa Recta:** Acts as a **countercurrent exchanger**, maintaining the gradient without washing it out. * **ADH Source:** Synthesized in the **Supraoptic nucleus** (primarily) and Paraventricular nucleus of the hypothalamus; stored in the posterior pituitary.

Q: Which of the following substances is primarily filtered with little secretion or reabsorption in the renal tubules?

Creatinine. **Explanation:** The correct answer is **Creatinine**. This question tests the fundamental concept of how the kidney handles different solutes based on filtration, reabsorption, and secretion. **1. Why Creatinine is Correct:** Creatinine is an endogenous breakdown product of creatine phosphate in muscles. In the nephron, it is **freely filtered** by the glomerulus and undergoes **minimal tubular secretion** (about 10–20%) with **zero reabsorption**. Because the amount excreted in urine is nearly equal to the amount filtered, it serves as a practical clinical marker for estimating the Glomerular Filtration Rate (GFR). **2. Why the Other Options are Incorrect:** * **Sodium (A):** Sodium is freely filtered but extensively **reabsorbed** (approx. 99%) throughout the nephron (Proximal Tubule, Loop of Henle, and Distal Tubule) to maintain fluid and electrolyte balance. * **Glucose (C) and Amino Acids (D):** These are essential nutrients. They are freely filtered but are **completely reabsorbed** (100%) in the Proximal Convoluted Tubule (PCT) via secondary active transport (SGLT transporters for glucose). They should not appear in the urine under normal physiological conditions. **Clinical Pearls for NEET-PG:** * **Inulin:** The "Gold Standard" for measuring GFR because it is freely filtered and undergoes **neither** secretion nor reabsorption. * **Creatinine Clearance:** Slightly **overestimates** GFR because of the small amount of tubular secretion. * **Para-aminohippuric acid (PAH):** Used to measure **Renal Plasma Flow (RPF)** because it is filtered and then almost entirely secreted, resulting in near-total clearance from the blood in a single pass. * **Transport Maximum ($T_m$):** Glucose begins to appear in urine (glycosuria) when blood glucose exceeds the renal threshold of approximately **180 mg/dL**.

Q: Which of the following statements is true regarding oliguria?

Urine formation of less than 500 mL/24 hours. ### Explanation **Concept of Obligatory Urine Volume** The correct answer is **C (Urine formation of less than 500 mL/24 hours)**. This definition is rooted in the concept of **obligatory urine volume**. To maintain homeostasis, an average adult must excrete approximately 600 mOsm of metabolic waste (solutes) daily. Since the maximum concentrating capacity of the human kidney is about 1200 mOsm/L, the minimum volume of water required to flush these solutes out is: * $600\text{ mOsm} \div 1200\text{ mOsm/L} = 0.5\text{ L (or 500 mL)}$ Therefore, any volume below 500 mL/day is insufficient to clear metabolic waste, leading to **azotemia** (elevation of nitrogenous products). **Analysis of Incorrect Options:** * **Option A (< 50 mL/day):** This defines **Anuria**. Clinically, anuria is often defined as <100 mL/day, but <50 mL is the strict threshold used to indicate total suppression of urine or complete urinary tract obstruction. * **Option B ( 3 L/day (seen in Diabetes Mellitus and Diabetes Insipidus). * **Normal Urine Output:** 0.5 to 2.0 mL/kg/hour. * **Pediatric Oliguria:** Defined as 2%.

Q: Non-oliguric renal failure is caused by damage to which part of the nephron?

Collecting duct. **Explanation:** **1. Why the Collecting Duct is Correct:** Non-oliguric renal failure (NORF) is a clinical state where the kidneys fail to concentrate urine despite a decrease in the Glomerular Filtration Rate (GFR). The **Collecting Duct (CD)** is the primary site for final urine concentration under the influence of Antidiuretic Hormone (ADH). Damage to the collecting duct cells (often due to nephrotoxins like aminoglycosides or partial obstruction) results in a loss of responsiveness to ADH. Consequently, the kidney cannot reabsorb water, leading to a high output of dilute urine (polyuria or normal volume) despite failing excretory functions. **2. Why the Other Options are Incorrect:** * **Afferent & Efferent Arterioles (A & B):** Damage to these vessels primarily affects the GFR and renal blood flow. Significant damage here usually leads to **oliguric** renal failure because the initial filtration pressure drops sharply, reducing urine formation at the source. * **Ascending Limb of the Loop of Henle (C):** While this segment is crucial for the countercurrent multiplier system, damage here typically presents as specific electrolyte wasting (like Bartter-like syndromes). While it affects concentration, the definitive "non-oliguric" failure pattern is most classically associated with distal tubular and collecting duct dysfunction. **3. NEET-PG High-Yield Pearls:** * **Definition:** Non-oliguric renal failure is defined as acute kidney injury (AKI) with a urine output **>400 ml/day**. * **Common Causes:** Aminoglycoside toxicity (e.g., Gentamicin), Amphotericin B, and contrast-induced nephropathy are classic triggers for non-oliguric AKI. * **Prognosis:** Non-oliguric AKI generally has a **better prognosis** and lower mortality rate compared to oliguric AKI. * **Key Concept:** In NORF, the "quality" of filtration is lost (waste retention) even if the "quantity" of urine remains normal.

Q: Which of the following is not a part of the glomerular filtration barrier?

Mesangial cell. The **glomerular filtration barrier (GFB)** is a highly specialized three-layered structure responsible for the ultrafiltration of blood while preventing the passage of plasma proteins. ### Why Mesangial Cells are the Correct Answer **Mesangial cells** are located between the glomerular capillaries but are **not** part of the filtration interface itself. Their primary functions include providing structural support to the capillary loops, secreting the extracellular matrix, and performing phagocytosis to remove trapped macromolecules. While they can influence the glomerular filtration rate (GFR) by contracting in response to Angiotensin II (thereby reducing surface area), they do not form a layer of the barrier through which filtrate must pass. ### Analysis of Incorrect Options (Components of the GFB) 1. **Endothelial cells (Option B):** The innermost layer. These are **fenestrated** capillaries (pores of 70–100 nm) that prevent the passage of blood cells but allow most plasma components through. 2. **Basement membrane (Option D):** The middle layer. It is composed of Type IV collagen and heparan sulfate proteoglycans. It acts as a **size and charge barrier** (negatively charged), repelling proteins like albumin. 3. **Podocytes (Option C):** The outermost layer (visceral epithelium). They possess foot processes (pedicels) that interdigitate to form **filtration slits** bridged by a diaphragm containing the protein **nephrin**. ### High-Yield Clinical Pearls for NEET-PG * **Charge Selectivity:** The GFB is negatively charged due to sialoglycoproteins and heparan sulfate. Loss of this negative charge (e.g., in **Minimal Change Disease**) leads to selective albuminuria. * **Nephrin:** Mutations in the gene encoding nephrin (*NPHS1*) result in **Finnish-type congenital nephrotic syndrome**. * **Goodpasture Syndrome:** Characterized by antibodies against the alpha-3 chain of Type IV collagen in the glomerular basement membrane.

Q: Which of the following ions is least absorbed in the renal tubules?

Creatinine. **Explanation:** The renal handling of substances depends on the balance between filtration, reabsorption, and secretion. The correct answer is **Creatinine** because it is the only substance among the options that undergoes **zero reabsorption** in the renal tubules. **1. Why Creatinine is Correct:** Creatinine is a metabolic byproduct of muscle metabolism. Once filtered at the glomerulus, it is **not reabsorbed** by the tubules. In fact, a small amount is actively secreted into the lumen. Because the amount excreted in urine is slightly more than the amount filtered, creatinine clearance is used as a clinical marker to estimate the Glomerular Filtration Rate (GFR). **2. Why the Other Options are Incorrect:** * **Sodium (A):** Sodium is the most actively reabsorbed ion. Approximately **99%** of filtered sodium is reabsorbed throughout the nephron (65% in the proximal tubule) to maintain fluid and electrolyte balance. * **Urea (B):** Urea is a waste product, but it is still partially reabsorbed. About **40-50%** of filtered urea is reabsorbed passively, primarily in the proximal tubule and medullary collecting ducts (where it contributes to the medullary osmotic gradient). * **Glucose (D):** Under normal physiological conditions, **100%** of filtered glucose is reabsorbed in the Proximal Convoluted Tubule (PCT) via SGLT2 and SGLT1 transporters. It only appears in urine if the blood glucose level exceeds the renal threshold (~180 mg/dL). **High-Yield Clinical Pearls for NEET-PG:** * **Inulin:** The "gold standard" for GFR measurement because it is freely filtered and undergoes **neither reabsorption nor secretion**. * **PAH (Para-aminohippurate):** Used to measure Renal Plasma Flow (RPF) because it is filtered and almost entirely secreted, resulting in near-total clearance. * **Renal Threshold for Glucose:** 180 mg/dL; **Transport Maximum (TmG):** 375 mg/min in men, 300 mg/min in women.

Q: In the absence of vasopressin, what is the greatest fraction of filtered water absorbed?

Proximal tubule.. **Explanation:** The correct answer is **Proximal tubule (A)**. The absorption of water in the nephron is divided into two types: **obligatory** and **facultative**. 1. **Proximal Convoluted Tubule (PCT):** Approximately **65-70%** of filtered water is reabsorbed here regardless of the body's hydration status or the presence of Antidiuretic Hormone (ADH/Vasopressin). This is "obligatory" water reabsorption, driven by the active transport of sodium (iso-osmotic reabsorption). Even in the complete absence of vasopressin (e.g., Diabetes Insipidus), the PCT remains the site where the largest fraction of water is reclaimed. **Why other options are incorrect:** * **Loop of Henle (B):** Reabsorbs about 15% of filtered water, primarily in the thin descending limb. The ascending limb is impermeable to water. * **Distal Tubule (C) & Cortical Collecting Duct (D):** These segments are responsible for "facultative" water reabsorption. Their permeability to water is strictly dependent on **Vasopressin (ADH)** acting on V2 receptors to insert Aquaporin-2 channels. In the absence of vasopressin, these segments become virtually impermeable to water, leading to the excretion of large volumes of dilute urine. **High-Yield Clinical Pearls for NEET-PG:** * **Obligatory Water Reabsorption:** Occurs in the PCT (65%) and Descending Loop of Henle (15%). Total = ~80%. * **Facultative Water Reabsorption:** Occurs in the Late DT and Collecting Ducts (remaining ~20%) under ADH control. * **Descending Limb of Henle:** Permeable to water but impermeable to solutes (concentrating segment). * **Ascending Limb of Henle:** Impermeable to water but active in solute reabsorption (diluting segment). * **Maximum Urine Osmolality:** 1200–1400 mOsm/L (in the presence of ADH). * **Minimum Urine Osmolality:** 30–50 mOsm/L (in the absence of ADH).

Question 1

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

Accepted Answer

Increased glomerular capillary filtration coefficient

Answer

Increased afferent arteriolar resistance

Answer

Decreased efferent arteriolar resistance

Answer

Increased Bowman's capsule hydrostatic pressure

Question 2

A 35-year-old man has fasting and postprandial blood sugar within normal limits, but his urine sugar is 3 plus (+++). What is the most likely diagnosis?

Accepted Answer

Renal glycosuria

Answer

Pancreatic insufficiency

Answer

Alimentary glycosuria

Answer

High carbohydrate diet taken in the morning

Question 3

What is the maximum osmolarity of urine in the presence of ADH?

Accepted Answer

1200 mOsmol / L

Answer

50 mOsmol / L

Answer

800 mOsmol / L

Answer

500 mOsmol / L

Question 4

Which of the following substances is primarily filtered with little secretion or reabsorption in the renal tubules?

Accepted Answer

Creatinine

Answer

Sodium

Answer

Glucose

Answer

Amino acids

Question 5

Which of the following statements is true regarding oliguria?

Accepted Answer

Urine formation of less than 500 mL/24 hours

Answer

Urine formation less than 50 mL/ 24 hours

Answer

Urine formation less than 250 mL/ 24 hours

Answer

Urine formation less than 750 mL/24 hours

Question 6

Non-oliguric renal failure is caused by damage to which part of the nephron?

Accepted Answer

Collecting duct

Answer

Afferent arteriole

Answer

Efferent arteriole

Answer

Ascending limb of the Loop of Henle

Question 7

Which of the following is not a part of the glomerular filtration barrier?

Accepted Answer

Mesangial cell

Answer

Endothelial cell

Answer

Podocyte

Answer

Basement membrane

Question 8

Which of the following ions is least absorbed in the renal tubules?

Accepted Answer

Creatinine

Answer

Sodium

Answer

Urea

Answer

Glucose

Question 9

In the absence of vasopressin, what is the greatest fraction of filtered water absorbed?

Accepted Answer

Proximal tubule.

Answer

Loop of Henle.

Answer

Distal tubule.

Answer

Cortical collecting duct.

Question 10

Glomerular filtration rate (GFR) is determined by?

Accepted Answer

All of the above

Answer

Afferent arteriolar resistance

Answer

Efferent arteriolar resistance

Answer

Mean arterial pressure

Renal Physiology — MCQs

Renal Physiology — MCQs

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