Renal Physiology Practice Questions

Q: Which cells in the kidney produce 1,25-dihydroxycholecalciferol?

Proximal convoluted tubule cells. The kidney plays a vital role in mineral homeostasis by performing the final step in the activation of Vitamin D. ### **Explanation of the Correct Answer** The correct answer is **B. Proximal convoluted tubule (PCT) cells**. The enzyme **1-alpha-hydroxylase** is primarily located in the mitochondria of the PCT cells. This enzyme converts 25-hydroxycholecalciferol (calcidiol) into **1,25-dihydroxycholecalciferol (calcitriol)**, which is the most active form of Vitamin D. This process is tightly regulated by Parathyroid Hormone (PTH), which stimulates the enzyme in response to low serum calcium or phosphate levels. ### **Analysis of Incorrect Options** * **A. Granular cells:** These are modified smooth muscle cells located in the afferent arterioles. Their primary function is the synthesis, storage, and release of **renin**, not Vitamin D activation. * **C. Mesangial cells:** These provide structural support to the glomerular capillaries and have contractile properties to regulate the glomerular filtration rate (GFR). * **D. Peritubular cells:** Specifically, the peritubular interstitial cells (fibroblasts) in the renal cortex are the primary site for the production of **Erythropoietin (EPO)** in response to hypoxia. ### **High-Yield Clinical Pearls for NEET-PG** * **Chronic Kidney Disease (CKD):** Loss of PCT mass leads to calcitriol deficiency, resulting in secondary hyperparathyroidism and renal osteodystrophy. * **Regulation:** 1-alpha-hydroxylase is **stimulated by PTH** and low phosphate, but **inhibited by FGF-23** and high calcitriol levels (negative feedback). * **Site of 25-hydroxylation:** Occurs in the **Liver** (via 25-hydroxylase), while 1-alpha-hydroxylation occurs in the **Kidney**.

Q: What is the principal site of sodium absorption in the nephron?

Proximal convoluted tubule. **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the "workhorse" of the nephron. It is the principal site for the reabsorption of the majority of the glomerular filtrate. Approximately **65-70% of filtered sodium (Na+)** is reabsorbed here, primarily through the Na+/H+ exchanger (NHE3) and co-transport with glucose, amino acids, and phosphate. This reabsorption is iso-osmotic, meaning water follows sodium passively to maintain osmotic balance. **Analysis of Incorrect Options:** * **Loop of Henle (Thick Ascending Limb):** This segment reabsorbs about **20-25%** of filtered sodium via the **Na+-K+-2Cl- cotransporter (NKCC2)**. It is the site of action for loop diuretics (e.g., Furosemide). * **Distal Convoluted Tubule (DCT):** This segment reabsorbs only about **5-8%** of sodium via the **Na+-Cl- symporter**. It is the site of action for Thiazide diuretics. * **Collecting Duct:** This is the final site for "fine-tuning" sodium balance, accounting for only **2-3%** of reabsorption. Sodium entry here occurs through **ENaC (Epithelial Sodium Channels)**, which are regulated by Aldosterone. **High-Yield Clinical Pearls for NEET-PG:** * **Obligatory Reabsorption:** Reabsorption in the PCT is "obligatory" and independent of hormonal control, unlike the distal segments. * **Glucose Reabsorption:** 100% of filtered glucose is reabsorbed in the PCT (via SGLT2/SGLT1) unless the renal threshold (180 mg/dL) is exceeded. * **Carbonic Anhydrase:** The PCT is the site of action for Acetazolamide, which inhibits sodium bicarbonate reabsorption. * **Smallest fraction of Na+ reabsorption:** Occurs in the medullary collecting ducts.

Q: In which region of the nephron are P cells (principal cells) located?

Collecting duct. **Explanation:** The correct answer is **D. Collecting duct.** **1. Why the Collecting Duct is Correct:** Principal cells (P cells) are the predominant cell type found in the **late distal tubule** and throughout the **collecting duct**. Their primary physiological role is the regulation of water and electrolyte balance under hormonal control: * **Sodium (Na+) Reabsorption:** They reabsorb Na+ via Epithelial Sodium Channels (ENaC). * **Potassium (K+) Secretion:** They are the main site for K+ secretion into the tubular lumen. * **Water Reabsorption:** They express Aquaporin-2 (AQP2) receptors, which are regulated by **Antidiuretic Hormone (ADH/Vasopressin)**. * **Aldosterone Action:** Aldosterone acts primarily on P cells to increase Na+ reabsorption and K+ secretion. **2. Why Other Options are Incorrect:** * **A. PCT:** This region is lined by cells with a prominent brush border (microvilli) for bulk reabsorption of glucose, amino acids, and ions. It does not contain P cells. * **B. Loop of Henle:** The thin segments consist of simple squamous epithelium, while the Thick Ascending Limb (TAL) contains cuboidal cells specialized for the Na+-K+-2Cl- cotransporter (NKCC2). * **C. DCT:** The early DCT contains "DCT cells" specialized for the Na+-Cl- symporter (NCC). While P cells begin to appear in the late (connecting) tubule, the collecting duct is their definitive and most characteristic location. **3. High-Yield Clinical Pearls for NEET-PG:** * **Intercalated Cells (I cells):** Also found in the collecting duct, these are responsible for acid-base balance (**Type A** secretes H+; **Type B** secretes HCO3-). * **Liddle’s Syndrome:** Caused by overactivity of ENaC channels in P cells, leading to hypertension and hypokalemia. * **Potassium-Sparing Diuretics:** Amiloride and Triamterene work by blocking ENaC channels in the P cells, while Spironolactone antagonizes the aldosterone receptor within these same cells.

Q: What is the exact number of solutes transported in the thick ascending limb of the Loop of Henle per carrier pump?

Transports one Na+, one K+, and two Cl-.. ### Explanation The correct answer is **A: Transports one Na+, one K+, and two Cl-**. **Underlying Concept:** The thick ascending limb (TAL) of the Loop of Henle is known as the "diluting segment" of the nephron. The primary transport mechanism on the apical (luminal) membrane is the **NKCC2 symporter** (Sodium-Potassium-2-Chloride co-transporter). This carrier protein moves four ions simultaneously into the cell: **one sodium (Na+), one potassium (K+), and two chloride (Cl-) ions**. This process is **electroneutral**, meaning the net charge moved across the membrane by the pump itself is zero (1+ + 1+ - 2 = 0). This transporter utilizes the sodium gradient created by the basolateral Na+/K+ ATPase to drive the secondary active transport of these solutes. **Analysis of Incorrect Options:** * **Options B, C, and D:** These are incorrect because they misstate the stoichiometry of the NKCC2 pump. Any ratio other than 1:1:2 would violate the electroneutrality of this specific symporter and does not align with the physiological structure of the SLC12A1 protein (the gene encoding NKCC2). **High-Yield Clinical Pearls for NEET-PG:** * **Loop Diuretics:** Drugs like **Furosemide** and Torsemide act by specifically inhibiting the NKCC2 symporter. * **Bartter Syndrome:** A genetic defect in the NKCC2 transporter (Type 1) or the ROMK channel (Type 2) mimics the effect of chronic loop diuretic use, leading to hypokalemia, metabolic alkalosis, and hypercalciuria. * **Lumen-Positive Potential:** While the pump is electroneutral, some K+ leaks back into the lumen via **ROMK channels**, creating a +10 to +20 mV potential. This "lumen-positive" charge is the driving force for the **paracellular reabsorption** of divalent cations like **Calcium (Ca2+) and Magnesium (Mg2+)**.

Q: Glucose is absorbed through the apical membrane of the proximal convoluted tubule by which transporter?

SGLT-2. **Explanation:** The reabsorption of glucose in the kidney occurs almost entirely in the **Proximal Convoluted Tubule (PCT)**. This process involves two distinct steps: transport across the apical (luminal) membrane and transport across the basolateral membrane. **Why SGLT-2 is correct:** Glucose is transported across the **apical membrane** against its concentration gradient via **Secondary Active Transport**. This is mediated by **SGLT-2** (Sodium-Glucose Co-transporter 2), which couples the movement of one sodium ion with one glucose molecule. SGLT-2 is a high-capacity, low-affinity transporter located in the early part (S1 segment) of the PCT, responsible for reabsorbing approximately **90%** of filtered glucose. **Analysis of Incorrect Options:** * **SGLT-1:** This is a low-capacity, high-affinity transporter located in the late PCT (S3 segment). It reabsorbs the remaining 10% of glucose. It is also the primary transporter for glucose absorption in the small intestine. * **GLUT-2:** This is a **Facilitated Diffusion** transporter located on the **basolateral membrane** of the early PCT. It moves glucose out of the cell into the interstitium. * **GLUT-1:** This transporter is located on the basolateral membrane of the late PCT (S3 segment), working in conjunction with SGLT-1. **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Threshold for Glucose:** Glucose begins to appear in the urine (glycosuria) when blood glucose levels exceed **180 mg/dL**. 2. **Tubular Transport Maximum ($T_m$):** The transport system becomes fully saturated at a plasma glucose level of approximately **375 mg/min** in men. 3. **SGLT-2 Inhibitors (Gliflozins):** Drugs like Dapagliflozin inhibit SGLT-2 and are used in treating Type 2 Diabetes to promote glucose excretion in urine.

Q: A 20-year-old male patient presents with a decrease in urinary output to about 400ml/day. Both renal blood flow (RBF) and glomerular filtration rate (GFR) were decreased. Which of the following causes a decrease in both RBF and GFR?

Constriction of the afferent arteriole. **Explanation:** The Glomerular Filtration Rate (GFR) and Renal Blood Flow (RBF) are primarily regulated by the resistance of the afferent and efferent arterioles. **Why Option C is Correct:** **Constriction of the afferent arteriole** increases resistance *before* the blood reaches the glomerulus. This leads to: 1. **Decreased RBF:** Increased total renal vascular resistance reduces blood flow into the kidney. 2. **Decreased GFR:** Reduced flow into the glomerular capillaries lowers the glomerular hydrostatic pressure ($P_{GC}$), which is the primary driving force for filtration. **Analysis of Incorrect Options:** * **Option A (Dilation of efferent arteriole):** Decreases resistance to outflow. This **increases RBF** (less resistance) but **decreases GFR** because the "back pressure" in the glomerulus drops. * **Option B (Dilation of afferent arteriole):** Decreases resistance to inflow. This **increases both RBF and GFR** (due to increased $P_{GC}$). * **Option D (Constriction of efferent arteriole):** Increases resistance to outflow. This **decreases RBF** but **increases GFR** (up to a point) because it increases the hydrostatic pressure ($P_{GC}$) within the glomerular capillaries. **High-Yield Clinical Pearls for NEET-PG:** * **NSAIDs** inhibit prostaglandins (which normally dilate the afferent arteriole), leading to **afferent constriction** and a drop in GFR. * **ACE Inhibitors** prevent the action of Angiotensin II (which preferentially constricts the efferent arteriole), leading to **efferent dilation** and a drop in GFR. * **Key Rule:** If both RBF and GFR move in the **same** direction, the primary change is at the **afferent** arteriole. If they move in **opposite** directions, the primary change is at the **efferent** arteriole.

Question 1

What would tend to decrease GFR by more than 10% in a normal kidney?

Accepted Answer

50% decrease in efferent arteriolar resistance

Answer

Decrease in mean arterial pressure from 100 to 85 mm Hg

Answer

50% decrease in afferent arteriolar resistance

Answer

50% increase in the glomerular capillary filtration coefficient

Question 2

Which cells in the kidney produce 1,25-dihydroxycholecalciferol?

Accepted Answer

Proximal convoluted tubule cells

Answer

Granular cells

Answer

Mesangial cells

Answer

Peritubular cells

Question 3

What is the principal site of sodium absorption in the nephron?

Accepted Answer

Proximal convoluted tubule

Answer

Distal convoluted tubule

Answer

Loop of Henle (thick portion)

Answer

Collecting duct

Question 4

Fractional excretion of sodium (FENa) is calculated as:

Accepted Answer

(Urine sodium / Serum sodium) / (Urine potassium / Serum potassium) x 100

Answer

(Urine sodium x Serum potassium) / (Serum sodium x Urine potassium) x 100

Answer

(Urine potassium x Serum sodium) / (Serum potassium x Urine sodium) x 100

Answer

(Urine sodium x Urine potassium) / (Serum sodium x Serum potassium) x 100

Question 5

In which region of the nephron are P cells (principal cells) located?

Accepted Answer

Collecting duct

Answer

Proximal Convoluted Tubule (PCT)

Answer

Loop of Henle

Answer

Distal Convoluted Tubule (DCT)

Question 6

What is the exact number of solutes transported in the thick ascending limb of the Loop of Henle per carrier pump?

Accepted Answer

Transports one Na+, one K+, and two Cl-.

Answer

Transports two Na+, two K+, and two Cl-.

Answer

Transports two Na+, one K+, and two Cl-.

Answer

Transports three Na+, two K+, and two Cl-.

Question 7

The glomerular filtration barrier does not permit which of the following molecular sizes?

Accepted Answer

40000 daltons

Answer

4000 daltons

Answer

400 daltons

Answer

40 daltons

Question 8

Which of the following statements is true regarding calcium reabsorption in the kidney?

Accepted Answer

The major regulating factor for calcium reabsorption is parathormone.

Answer

Most of the calcium reabsorption occurs in the distal convoluted tubule (DCT).

Answer

Parathormone decreases calcium reabsorption.

Answer

Increased plasma phosphate decreases calcium reabsorption.

Question 9

Glucose is absorbed through the apical membrane of the proximal convoluted tubule by which transporter?

Accepted Answer

SGLT-2

Answer

SGLT-1

Answer

GLUT-1

Answer

GLUT-2

Question 10

A 20-year-old male patient presents with a decrease in urinary output to about 400ml/day. Both renal blood flow (RBF) and glomerular filtration rate (GFR) were decreased. Which of the following causes a decrease in both RBF and GFR?

Accepted Answer

Constriction of the afferent arteriole

Answer

Dilation of the efferent arteriole

Answer

Dilation of the afferent arteriole

Answer

Constriction of the efferent arteriole

Renal Physiology — MCQs

Renal Physiology — MCQs

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