Renal Physiology Practice Questions

Q: Isoosmolar urine is seen in which of the following conditions?

Acute tubular necrosis (ATN). **Explanation:** The correct answer is **Acute Tubular Necrosis (ATN)**. **1. Why ATN is correct:** In ATN, the tubular epithelial cells are damaged, leading to a loss of the kidney's ability to concentrate or dilute urine. The kidneys cannot maintain the medullary osmotic gradient or respond to ADH. Consequently, the urine osmolality becomes fixed at the same osmolality as plasma (approximately **290–300 mOsm/kg**), a condition known as **Isosthenuria**. The Urine Specific Gravity typically remains fixed at **1.010**. **2. Why other options are incorrect:** * **Severe Dehydration:** In a healthy physiological response to dehydration, ADH levels rise, causing the kidneys to reabsorb water. This results in **hyperosmolar (concentrated) urine** (typically >500 mOsm/kg). * **Diabetes Insipidus (DI):** Whether central or nephrogenic, DI is characterized by an inability to reabsorb water in the collecting ducts. This leads to the excretion of large volumes of **hypoosmolar (dilute) urine** (typically <200 mOsm/kg). * **Polycystic Kidney Disease (PCKD):** While chronic kidney diseases eventually lead to a loss of concentrating ability, in the early to mid-stages, the urine is not characteristically isoosmolar in the same acute, diagnostic fashion as seen in ATN. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isosthenuria:** Urine Specific Gravity of **1.010** is the hallmark of ATN. * **Fractional Excretion of Sodium (FeNa):** In ATN, FeNa is typically **>2%** (due to tubular damage), whereas in Pre-renal Azotemia, it is **<1%** (tubules are intact and conserving sodium). * **Urinary Casts:** Look for **"Muddy brown granular casts"** in the urine sediment, which are pathognomonic for ATN.

Q: Which of the following conditions leads to a decrease in Glomerular Filtration Rate (GFR)?

Hypotension. **Explanation:** The Glomerular Filtration Rate (GFR) is determined by the balance of Starling forces across the glomerular capillaries. The formula for GFR is: **GFR = Kf × [(Pgc – Pbs) – (πgc – πbs)]**. **1. Why Hypotension is Correct:** Hypotension leads to a decrease in **Glomerular Capillary Hydrostatic Pressure (Pgc)**. Since Pgc is the primary driving force for filtration, a significant drop in systemic blood pressure (below the autoregulatory range of 80–180 mmHg) reduces the pressure gradient available to push fluid into Bowman’s space, thereby decreasing GFR. **2. Why the other options are incorrect:** * **Hypoproteinemia:** This condition involves a decrease in plasma proteins (e.g., albumin), which lowers the **Plasma Colloid Osmotic Pressure (πgc)**. Since πgc is a force that opposes filtration, a decrease in this pressure actually **increases GFR**. * **Diuretics:** Most diuretics act by inhibiting ion reabsorption in the tubules (e.g., Loop diuretics in the Thick Ascending Limb). While they increase urine output, they do not acutely decrease GFR as a primary mechanism; in fact, some (like Mannitol) may transiently increase GFR by expanding extracellular volume. **Clinical Pearls for NEET-PG:** * **Autoregulation:** GFR remains constant between mean arterial pressures of **80–180 mmHg** due to the Myogenic mechanism and Tubuloglomerular Feedback (TGF). * **Afferent vs. Efferent:** Constriction of the afferent arteriole decreases GFR, while moderate constriction of the efferent arteriole increases GFR. * **Gold Standard Marker:** Inulin clearance is the gold standard for measuring GFR because it is freely filtered but neither reabsorbed nor secreted.

Q: What is the normal plasma osmolality in mOsmol/kg?

285-295. **Explanation:** The correct answer is **C (285–295 mOsmol/kg)**. Plasma osmolality is a measure of the concentration of solutes (primarily sodium, chloride, bicarbonate, glucose, and urea) in the blood. In a healthy individual, the body maintains this value within a narrow range of **285–295 mOsmol/kg** to ensure cellular stability. This homeostasis is primarily regulated by the **hypothalamic-pituitary-renal axis** through the action of Antidiuretic Hormone (ADH) and the thirst mechanism. **Why the other options are incorrect:** * **Options A & B (240–275):** These values represent **hypo-osmolality**. Such levels are seen in states of water excess or significant hyponatremia (e.g., SIADH), which can lead to cerebral edema as water shifts into cells. * **Option D (300–312):** These values represent **hyper-osmolality**. This occurs in dehydration, diabetes insipidus, or hyperglycemia. A plasma osmolality above 300 mOsmol/kg is a potent stimulus for ADH release and the sensation of thirst. **High-Yield Facts for NEET-PG:** 1. **Calculated Osmolality Formula:** $2[Na^+] + \frac{\text{Glucose}}{18} + \frac{BUN}{2.8}$. Sodium is the primary determinant of plasma osmolality. 2. **Osmolar Gap:** The difference between measured and calculated osmolality. A gap $>10$ suggests the presence of unmeasured osmoles (e.g., Ethanol, Methanol, Ethylene glycol). 3. **Osmoreceptors:** Located in the **OVLT** (Organum Vasculosum of the Lamina Terminalis) and the **SFO** (Subfornical Organ) of the hypothalamus; they sense changes as small as 1%.

Q: Which of the following substances relaxes mesangial cells in the glomerulus?

Dopamine. **Explanation:** The glomerular mesangial cells are specialized contractile cells located between the capillary loops. Their primary function is to regulate the **Glomerular Filtration Rate (GFR)** by altering the available surface area for filtration. When these cells contract, the surface area decreases (reducing GFR); when they relax, the surface area increases (raising GFR). **1. Why Dopamine is Correct:** Dopamine acts on **D1 receptors** in the kidney to cause vasodilation and **relaxation of mesangial cells**. This relaxation increases the effective filtration surface area, contributing to the increase in GFR and natriuresis (sodium excretion) typically seen with low-to-moderate dose dopamine infusions. Other substances that cause relaxation include **Atrial Natriuretic Peptide (ANP)**, cAMP, and Prostaglandin E2 (PGE2). **2. Why the Other Options are Incorrect:** * **Histamine:** Acts as a potent **contractor** of mesangial cells, thereby reducing the filtration surface area. * **Angiotensin II & III:** These are powerful vasoconstrictors. Angiotensin II is the primary hormonal regulator that causes mesangial **contraction** via AT1 receptors. Angiotensin III has similar, though less potent, effects. * **Platelet-derived growth factor (PDGF):** This is a mitogen that stimulates mesangial cell proliferation and **contraction**. It plays a significant role in the pathophysiology of many glomerular diseases (e.g., glomerulonephritis). **High-Yield Clinical Pearls for NEET-PG:** * **Contraction (Decreases GFR):** Angiotensin II, Vasopressin (ADH), Endothelin, Histamine, Noradrenaline, and PAF. * **Relaxation (Increases GFR):** Dopamine, ANP, PGE2, and cAMP. * **Mesangial Function:** They also provide structural support to capillaries and possess phagocytic properties to remove macromolecules from the basement membrane.

Q: Which of the following is NOT a component of the countercurrent multiplier mechanism?

Vasa recta. The renal medullary osmotic gradient is maintained by two distinct systems: the **Countercurrent Multiplier** and the **Countercurrent Exchanger**. ### Why "Vasa Recta" is the Correct Answer The **Vasa recta** functions as a **Countercurrent Exchanger**, not a multiplier. Its role is passive; it prevents the dissipation of the medullary osmotic gradient by removing excess water and solutes without washing away the hypertonicity. It maintains the gradient created by the loop of Henle. ### Analysis of Other Options (The Multipliers) The Countercurrent Multiplier is an active process occurring in the **Loop of Henle** and the **Collecting Duct**: * **Thick Ascending Limb (TAL):** The "engine" of the multiplier. It actively pumps Na⁺, K⁺, and Cl⁻ into the interstitium via the NKCC2 transporter but is impermeable to water. * **Thin Descending Limb:** This segment is highly permeable to water but impermeable to solutes. Water leaves the tubule due to the high interstitial osmolarity created by the TAL. * **Collecting Duct:** Contributes to the multiplier through **Urea Recycling**. Under the influence of ADH, urea moves into the medullary interstitium, accounting for nearly 50% of the hypertonicity. ### High-Yield NEET-PG Pearls * **The "Single Effect":** This refers to the active transport of NaCl out of the TAL, which creates a 200 mOsm/L gradient between the tubule and the interstitium at any given level. * **NKCC2 Transporter:** Targeted by **Loop Diuretics** (e.g., Furosemide), which abolish the medullary gradient and result in dilute urine. * **Vasa Recta Blood Flow:** It is characteristically slow (only ~1-2% of total renal blood flow) to ensure the medullary gradient is not "washed out."

Question 1

Erythropoietin is produced by which of the following?

Accepted Answer

Interstitial cells of the peritubular capillary bed of the kidneys

Answer

Juxtaglomerular cells

Answer

Pars recta of PCT

Answer

Macula densa

Question 2

What is true regarding neutral substances with a size of 6 nm?

Accepted Answer

Permeability is inversely proportional to diameter

Answer

Freely filtered

Answer

Not filtered

Answer

None of the above

Question 3

Isoosmolar urine is seen in which of the following conditions?

Accepted Answer

Acute tubular necrosis (ATN)

Answer

Severe dehydration

Answer

Diabetes insipidus

Answer

Polycystic kidney disease (PCKD)

Question 4

Aldosterone is known to cause sodium retention. Its sodium-retaining action is exerted on which part of the nephron?

Accepted Answer

Collecting ducts

Answer

Proximal convoluted tubule

Answer

Ascending limb of loop of Henle

Answer

Early distal convoluted tubule

Question 5

All of the following are true about the macula densa except:

Accepted Answer

It is a sensor for glomerulotubular balance.

Answer

Na+ and Cl- enter the macula densa cells via the Na-K-2Cl cotransporter in their apical membranes.

Answer

Increased formation and release of adenosine occurs in macula densa cells.

Answer

It causes vasoconstriction of the afferent arteriole.

Question 6

Which of the following conditions leads to a decrease in Glomerular Filtration Rate (GFR)?

Accepted Answer

Hypotension

Answer

Hypoproteinemia

Answer

Diuretics

Answer

All of the above

Question 7

Liddle syndrome is caused by a genetic defect in which of the following?

Accepted Answer

ENaC (epithelial sodium channel) in the collecting duct/tubule

Answer

Na-K-2Cl co-transporter in the thick ascending limb of the loop of Henle

Answer

Na-Cl cotransporter in the distal tubule

Answer

Aquaporin 2 (AQP2) channel in the collecting duct

Question 8

What is the normal plasma osmolality in mOsmol/kg?

Accepted Answer

285-295

Answer

240-255

Answer

260-275

Answer

300-312

Question 9

Which of the following substances relaxes mesangial cells in the glomerulus?

Accepted Answer

Dopamine

Answer

Histamine

Answer

Angiotensin III

Answer

Platelet-derived growth factor (PDGF)

Question 10

Which of the following is NOT a component of the countercurrent multiplier mechanism?

Accepted Answer

Vasa recta

Answer

Thick ascending limb of the loop of Henle

Answer

Thin descending limb of the loop of Henle

Answer

Collecting duct

Renal Physiology — MCQs

Renal Physiology — MCQs

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