Renal Physiology Practice Questions

Q: The tubuloglomerular feedback is mediated by:

Sensing NaCl concentration in the macula densa. **Explanation:** **Tubuloglomerular Feedback (TGF)** is an intrinsic autoregulatory mechanism of the kidney where the **Macula Densa** (specialized cells in the Thick Ascending Limb) senses changes in the tubular fluid to regulate the Glomerular Filtration Rate (GFR). **Why Option C is Correct:** The primary sensor for TGF is the **NKCC2 transporter** (Na+-K+-2Cl- cotransporter) located on the apical membrane of macula densa cells. While both Na+ and Cl- are transported, research indicates that the macula densa specifically monitors the **total NaCl concentration**. When GFR increases, more NaCl reaches the macula densa. This leads to increased NaCl uptake, triggering the release of **Adenosine**, which causes vasoconstriction of the **Afferent Arteriole**, thereby normalizing GFR. **Analysis of Incorrect Options:** * **Options A & B:** While Na+ and Cl- are both sensed, they are sensed together via the NKCC2 transporter. Isolating one over the other is physiologically inaccurate as the transporter requires the presence of both ions to function. * **Option D:** TGF involves the opening of **stretch-activated non-specific cation channels** and calcium-activated channels, but the primary effector mechanism in the afferent arteriole is mediated by **Adenosine (A1 receptors)** and an increase in intracellular Calcium, not voltage-gated Na+ channels. **High-Yield NEET-PG Pearls:** * **The "Sensor":** Macula Densa. * **The "Effector":** Afferent Arteriole (Vasoconstriction). * **The "Mediator":** Adenosine (ATP is the precursor). * **Clinical Correlation:** Loop diuretics (e.g., Furosemide) inhibit the NKCC2 transporter, effectively "blunting" the TGF mechanism. * **Opposite Effect:** A decrease in NaCl at the macula densa stimulates the **Juxtaglomerular cells** to release **Renin**, activating the RAAS pathway.

Q: Parathormone (PTH) increases calcium levels and reduces phosphate levels by acting on which part of the nephron?

All of the above. **Explanation:** Parathormone (PTH) is the primary regulator of calcium and phosphate homeostasis. Its action on the kidney is multi-segmental, aimed at increasing serum calcium and decreasing serum phosphate. 1. **Proximal Tubule (PT):** This is the primary site for phosphate regulation. PTH binds to its receptors and inhibits the **Na⁺-PO₄³⁻ cotransporter (NPT2a)**, leading to phosphaturia (decreased phosphate levels). It also stimulates **1-alpha-hydroxylase** here, converting Vitamin D to its active form. 2. **Distal Convoluted Tubule (DCT):** This is the major site for PTH-mediated **calcium reabsorption**. PTH increases the opening of apical calcium channels (TRPV5), effectively raising serum calcium levels. 3. **Thick Ascending Limb (TAL):** PTH enhances the activity of the **Na⁺-K⁺-2Cl⁻ symporter**, which increases the positive luminal potential, thereby driving the paracellular reabsorption of calcium and magnesium. **Why "All of the above" is correct:** While textbooks often emphasize the PT for phosphate and the DCT for calcium, PTH receptors are present across all these segments. It acts on the PT to decrease phosphate and on the TAL and DCT to increase calcium reabsorption. Therefore, its net effect on the nephron involves all three segments. **Clinical Pearls for NEET-PG:** * **Mnemonic for PTH:** "P"TH = **P**hosphate **T**rash (increases phosphate excretion in the urine). * **Site of Action:** The most potent site for calcium reabsorption is the DCT, but the TAL also contributes significantly. * **Cyclic AMP:** PTH action on the proximal tubule increases urinary cAMP, which is a clinical marker for PTH activity. * **Hypoparathyroidism:** Results in hypocalcemia and hyperphosphatemia due to the loss of these renal actions.

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: All of the following statements concerning Atrial Natriuretic Peptide (ANP) are true EXCEPT?

Its effects are mediated by increased cyclic AMP.. **Explanation:** Atrial Natriuretic Peptide (ANP) is a powerful hormone secreted by the atrial myocytes in response to atrial stretch, serving as the body’s natural defense against fluid overload. **Why Option D is the correct answer (The Exception):** The cellular mechanism of ANP involves binding to its receptor (NPR-A), which has intrinsic **guanylyl cyclase** activity. This leads to the conversion of GTP to **cyclic GMP (cGMP)**, not cAMP. cGMP then activates protein kinase G (PKG) to mediate vasodilation and natriuresis. **Analysis of Incorrect Options:** * **Option A:** ANP is secreted in response to **increased blood volume** or pressure, which causes atrial wall stretch (mechanoreceptor activation). * **Option B:** ANP **increases GFR** through a "push-pull" mechanism: it dilates the afferent arteriole and constricts the efferent arteriole. This increases the glomerular capillary hydrostatic pressure. * **Option C:** ANP acts as an antagonist to the Renin-Angiotensin-Aldosterone System (RAAS). It **inhibits renin secretion** from juxtaglomerular cells and directly inhibits aldosterone secretion from the adrenal cortex. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Action:** ANP primarily acts on the **medullary collecting ducts** to inhibit Na+ reabsorption. * **Brain Natriuretic Peptide (BNP):** Secreted by ventricles; used clinically as a marker for **Heart Failure** (high sensitivity). * **Neprilysin:** The enzyme that degrades ANP/BNP. Neprilysin inhibitors (e.g., Sacubitril) are now used in heart failure management to prolong the beneficial effects of natriuretic peptides. * **Mnemonic:** ANP = **A**fferent **N**early **P**atent (Dilates), **E**fferent **E**xtremely **E**nded (Constricts).

Q: Uremia occurs when the total GFR is reduced by approximately what percentage?

25%. **Explanation:** The correct answer is **25% (Option A)**. This question tests the understanding of the kidney's functional reserve and the progression of Chronic Kidney Disease (CKD). **1. Why 25% is correct:** The kidneys possess a massive functional reserve. Clinical symptoms of **uremia** (a syndrome caused by the accumulation of nitrogenous waste products like urea and creatinine) typically do not manifest until the total Glomerular Filtration Rate (GFR) falls below **25% of its normal value** (roughly <30 mL/min/1.73m²). At this stage, the remaining functional nephrons can no longer compensate for the loss, leading to systemic toxicity, electrolyte imbalances, and fluid overload. **2. Why the other options are incorrect:** * **50% (Option B):** When GFR is reduced by 50%, the patient is often asymptomatic. This is seen in individuals who donate a kidney; the remaining kidney undergoes compensatory hypertrophy, and the patient does not develop uremia. * **60% (Option C):** A 60% reduction (leaving 40% function) corresponds to Stage 3 CKD. While biochemical abnormalities (like mild anemia or secondary hyperparathyroidism) may begin, overt uremic symptoms are rare. * **80% (Option D):** While uremia is certainly present at an 80% reduction (leaving 20% function), the question asks for the threshold at which it *occurs*. Uremia begins to manifest earlier, around the 25% functional mark. **High-Yield Clinical Pearls for NEET-PG:** * **Stages of CKD:** Stage 1 (>90 GFR), Stage 2 (60-89), Stage 3 (30-59), Stage 4 (15-29), Stage 5 (<15 or Dialysis). * **Uremic Frost:** A late clinical sign where urea crystallizes on the skin. * **Most common cause of death in Uremia:** Cardiovascular disease (not renal failure itself). * **First sign of CKD:** Often microalbuminuria, rather than a drop in GFR.

Question 1

The tubuloglomerular feedback is mediated by:

Accepted Answer

Sensing NaCl concentration in the macula densa

Answer

Sensing of Na+ concentration in the macula densa

Answer

Sensing of Cl- concentration in macula densa

Answer

Opening up of voltage-gated Na+ channels in afferent arteriole

Question 2

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 3

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 4

Parathormone (PTH) increases calcium levels and reduces phosphate levels by acting on which part of the nephron?

Accepted Answer

All of the above

Answer

Proximal tubules

Answer

Distal tubules

Answer

Thick ascending limb of Loop of Henle

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 is a non-osmotic stimulus for ADH secretion?

Accepted Answer

Hemorrhage

Answer

Uremia

Answer

Hyperglycemia

Answer

Excessive water ingestion

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

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 9

All of the following statements concerning Atrial Natriuretic Peptide (ANP) are true EXCEPT?

Accepted Answer

Its effects are mediated by increased cyclic AMP.

Answer

Its secretion is increased by an increase in blood volume.

Answer

It increases glomerular filtration rate.

Answer

It inhibits renin secretion.

Question 10

Uremia occurs when the total GFR is reduced by approximately what percentage?

Accepted Answer

25%

Answer

50%

Answer

60%

Answer

80%

Renal Physiology — MCQs

Renal Physiology — MCQs

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