Renal Physiology Practice Questions

Q: At what age does glomerular filtration rate typically begin to decline?

30-40 years. **Explanation:** The Glomerular Filtration Rate (GFR) reaches its peak adult levels (approximately 125 mL/min/1.73m²) by the age of 2 years. It remains relatively stable throughout young adulthood. However, starting between the ages of **30 and 40 years**, a progressive physiological decline in GFR begins. **Why 30-40 years is correct:** The decline is primarily due to the natural aging process of the kidneys, characterized by a gradual loss of functioning nephrons (nephrosclerosis), reduction in renal blood flow, and cortical atrophy. On average, the GFR decreases at a rate of approximately **0.8 to 1.0 mL/min/1.73m² per year** after the age of 40. **Analysis of Incorrect Options:** * **A (10-15 years):** At this stage, renal function is at its physiological peak. The kidneys are still growing or maintaining maximum efficiency. * **C & D (45-65 years):** While the decline becomes more clinically evident and measurable in these age groups, the *initiation* of the decline occurs much earlier (in the 4th decade of life). **High-Yield Clinical Pearls for NEET-PG:** * **Formula for Age-Related Decline:** A common rule of thumb is that GFR decreases by ~10 mL/min per decade after age 40. * **Creatinine Paradox:** Despite the decrease in GFR, serum creatinine levels often remain within the "normal" range in the elderly. This is because muscle mass (the source of creatinine) also decreases with age, masking the decline in renal clearance. * **Cockcroft-Gault Equation:** This formula specifically incorporates **age** as a variable in the denominator, reflecting its inverse relationship with GFR. * **Clinical Significance:** Dose adjustment for renally cleared drugs (e.g., Aminoglycosides, Digoxin) is crucial in elderly patients, even if their serum creatinine appears normal.

Q: What is the tonicity of the fluid in the distal convoluted tubule (DCT)?

Hypotonic. **Explanation:** The tonicity of the tubular fluid changes significantly as it passes through different segments of the nephron. The fluid entering the **Distal Convoluted Tubule (DCT)** is characteristically **hypotonic** (approximately 100 mOsm/L) compared to plasma (290 mOsm/L). **Why Option A is Correct:** This occurs because of the preceding action in the **Thick Ascending Limb (TAL)** of the Loop of Henle. The TAL is known as the **"diluting segment"** of the nephron. It actively reabsorbs solutes (via the $Na^+-K^+-2Cl^-$ symporter) but is strictly impermeable to water. As salt leaves the tubule without water following it, the luminal fluid becomes increasingly dilute. By the time it reaches the early DCT, the fluid is significantly more dilute than the surrounding interstitium and plasma. **Why Other Options are Incorrect:** * **Option B (Hypertonic):** Fluid is hypertonic only at the tip of the Loop of Henle (up to 1200 mOsm/L) due to the countercurrent multiplier system. * **Option C (Isotonic):** Fluid is isotonic (300 mOsm/L) in the **Proximal Convoluted Tubule (PCT)**, where water and solutes are reabsorbed in equal proportions (obligatory water reabsorption). **High-Yield Clinical Pearls for NEET-PG:** * **Early DCT:** Continues the dilution process via the **$Na^+-Cl^-$ cotransporter** (the target of **Thiazide diuretics**). It remains impermeable to water. * **Late DCT & Collecting Duct:** Tonicity here is **variable** and depends entirely on **Antidiuretic Hormone (ADH)**. In the presence of ADH, water is reabsorbed, making urine hypertonic; in its absence, urine remains hypotonic. * **Macula Densa:** Located at the junction of the TAL and DCT, it senses $Cl^-$ levels to regulate the Glomerular Filtration Rate (Tubuloglomerular feedback).

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.

Question 1

At what age does glomerular filtration rate typically begin to decline?

Accepted Answer

30-40 years

Answer

10-15 years

Answer

45-55 years

Answer

55-65 years

Question 2

In a patient with severe central diabetes insipidus caused by a lack of antidiuretic hormone secretion, which part of the nephron would have the lowest tubular fluid osmolarity?

Accepted Answer

Collecting duct

Answer

Proximal convoluted tubule

Answer

Loop of Henle (descending limb)

Answer

Loop of Henle (ascending limb)

Question 3

What is the tonicity of the fluid in the distal convoluted tubule (DCT)?

Accepted Answer

Hypotonic

Answer

Hypertonic

Answer

Isotonic

Answer

None of the above

Question 4

Angiotensin-II mainly acts by which mechanism on the glomerulus?

Accepted Answer

Efferent arteriole constriction

Answer

Afferent arteriole dilatation

Answer

Efferent arteriole dilatation

Answer

Afferent arteriole constriction

Question 5

Concentrating ability of the kidney is impaired if:

Accepted Answer

Blood flow through the vasa recta is increased substantially

Answer

There is damage to the glomerulus

Answer

There is excess secretion of ADH

Answer

There is vigorous activity of the Na+-K+-pump

Question 6

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 7

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 8

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 9

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 10

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.

Renal Physiology — MCQs

Renal Physiology — MCQs

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