Renal Physiology Practice Questions

Q: Which of the following substances undergoes the least reabsorption through the renal tubule?

Urea. **Explanation:** The renal tubule handles filtered substances through varying degrees of reabsorption and secretion to maintain homeostasis. The correct answer is **Urea** because it is the only substance among the options that is primarily treated as a waste product with significant passive excretion. * **Urea (Correct):** Approximately **40-50%** of filtered urea is reabsorbed, primarily in the proximal convoluted tubule (PCT) and the medullary collecting ducts (via UT-A1/A3 transporters). The remaining 50% is excreted. This is the highest excretion fraction among the listed options. * **Glucose:** Under normal physiological conditions, **100%** of filtered glucose is reabsorbed in the PCT via SGLT-2 and SGLT-1 transporters. It only appears in urine if the renal threshold (~180 mg/dL) is exceeded. * **Bicarbonate (HCO3):** About **80-90%** is reabsorbed in the PCT, and the remainder in the distal segments. It is vital for acid-base balance, and very little is excreted under normal conditions. * **Uric Acid:** Approximately **90%** of filtered uric acid is reabsorbed in the PCT (via URAT1). While it is a waste product, its reabsorption rate is significantly higher than that of urea. **High-Yield NEET-PG Pearls:** 1. **Creatinine:** Undergoes **zero reabsorption**; in fact, it is slightly secreted, making its clearance a slight overestimation of GFR. 2. **Urea Recycling:** Urea reabsorption in the medullary collecting ducts is essential for maintaining the **medullary osmotic gradient**, which allows for urine concentration. 3. **Splay:** This term refers to the appearance of glucose in the urine before the transport maximum ($T_m$) is reached due to the heterogeneity of nephrons.

Q: What is the renal threshold for glycosuria?

180 mg/dL. **Explanation:** The **Renal Threshold** for a substance is the plasma concentration at which the substance begins to appear in the urine. For glucose, this occurs when the filtered load exceeds the reabsorptive capacity of the proximal convoluted tubule (PCT). 1. **Why 180 mg/dL is correct:** Under normal physiological conditions, all filtered glucose is reabsorbed via **SGLT-2** (90%) and **SGLT-1** (10%) transporters. However, these transporters have a saturation point. While the theoretical maximum tubular reabsorptive capacity (**TmG**) is higher, glucose starts appearing in the urine at a venous plasma concentration of approximately **180 mg/dL**. This discrepancy between the theoretical TmG and the actual threshold is known as **"Splay,"** caused by the heterogeneity of nephrons and the kinetics of transporter binding. 2. **Analysis of Incorrect Options:** * **100 mg/dL:** This is the upper limit of normal fasting plasma glucose. At this level, the transporters are nowhere near saturation. * **300 mg/dL:** This is significantly higher than the threshold. By this stage, gross glycosuria is present, leading to osmotic diuresis. * **350 mg/dL:** This value represents the **Tubular Maximum (TmG)** for glucose (approx. 375 mg/min in men, 300 mg/min in women). This is the point where *all* nephrons have reached their maximum reabsorptive capacity. **High-Yield Clinical Pearls for NEET-PG:** * **Splay:** The curved portion of the glucose titration curve; it occurs because some nephrons have a lower reabsorptive capacity than others. * **Pregnancy:** The renal threshold for glucose **decreases** in pregnancy due to an increased Glomerular Filtration Rate (GFR), making glycosuria a common (though not always pathological) finding. * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** These drugs lower the renal threshold for glucose to treat Diabetes Mellitus by promoting therapeutic glycosuria.

Q: Which prostaglandin causes renal vasodilation?

PGE2. **Explanation:** The correct answer is **PGE2**. In the kidneys, prostaglandins play a vital role in maintaining renal hemodynamics, particularly during states of decreased effective circulating volume. **1. Why PGE2 is correct:** Prostaglandin E2 (PGE2) is a potent **renal vasodilator**. It acts primarily on the EP2 and EP4 receptors to relax vascular smooth muscle. Its key physiological role is to antagonize the vasoconstrictor effects of Angiotensin II and Norepinephrine on the afferent arterioles. By maintaining vasodilation, PGE2 ensures adequate Renal Blood Flow (RBF) and Glomerular Filtration Rate (GFR). It also inhibits sodium reabsorption in the thick ascending limb and water reabsorption in the collecting ducts (acting as a "natural diuretic"). **2. Analysis of Incorrect Options:** * **PGD2:** While PGD2 has vasodilatory properties in the systemic circulation (involved in flushing and allergic responses), it is not the primary prostaglandin responsible for regulating renal hemodynamics. * **PGF2α:** This prostaglandin is primarily a **vasoconstrictor** and causes contraction of smooth muscles, including the uterus (used for postpartum hemorrhage) and bronchioles. It does not cause renal vasodilation. **3. High-Yield NEET-PG Pearls:** * **NSAIDs and the Kidney:** NSAIDs inhibit COX enzymes, blocking the production of PGE2. In patients with heart failure or CKD, this loss of compensatory vasodilation can lead to **acute renal failure** due to unopposed vasoconstriction of the afferent arteriole. * **PGI2 (Prostacyclin):** Along with PGE2, PGI2 is the other major vasodilator in the kidney. If PGI2 were an option, it would also be correct. * **Triple Whammy:** Be wary of the combination of **ACE inhibitors + Diuretics + NSAIDs**, which significantly increases the risk of acute kidney injury by reducing GFR from multiple angles.

Q: Which of the following characteristics should an ideal substance for measuring Glomerular Filtration Rate (GFR) NOT possess?

Should be secreted in tubules. To calculate the **Glomerular Filtration Rate (GFR)** accurately, a substance must be filtered solely by the glomerulus. If a substance is **secreted** by the renal tubules, the amount appearing in the urine would be the sum of filtration *plus* secretion, leading to an **overestimation** of the GFR. Therefore, an ideal marker must be neither secreted nor reabsorbed by the tubules. ### Analysis of Options: * **A. Should be secreted in tubules (Correct):** This is a disqualifying characteristic. Secretion adds extra substance to the urine that did not pass through the glomerular filter, making the clearance value higher than the actual GFR. * **B. Should be non-toxic:** An ideal marker must be physiologically inert and safe for the patient, as it needs to be infused or present in the blood for measurement. * **C. Should not remain in the body:** The substance should not be metabolized or stored in the kidneys or other tissues; it must be excreted unchanged to ensure the plasma concentration reflects renal handling accurately. * **D. Should not be protein bound:** Large proteins cannot pass through the glomerular basement membrane. If a substance is protein-bound, it won't be filtered, making it impossible to measure GFR. ### High-Yield Pearls for NEET-PG: * **Inulin:** The **Gold Standard** for GFR measurement. It is a fructose polymer that is freely filtered but neither reabsorbed nor secreted. * **Creatinine:** The most common **clinical marker**. It is endogenous but slightly **secreted** by tubules, thus it overestimates GFR by about 10-20%. * **Para-aminohippuric acid (PAH):** Used to measure **Renal Plasma Flow (RPF)** because it is both filtered and almost completely secreted. * **Criteria for Ideal GFR Marker:** Freely filtered, not reabsorbed, not secreted, not metabolized, non-toxic, and has no effect on filtration rate.

Q: Given a urine flow rate of 10 ml/min, plasma inulin of 2 mg/ml, and urine inulin of 25 mg/ml, which of the following statements is true?

Inulin clearance equals Glomerular Filtration Rate (GFR).. **Explanation:** **1. Why Option A is Correct:** The core concept here is that **Inulin** is the "gold standard" substance for measuring the **Glomerular Filtration Rate (GFR)**. This is because Inulin is a fructose polymer that is freely filtered at the glomerulus but is **neither reabsorbed nor secreted** by the renal tubules. Therefore, the amount of inulin filtered per minute equals the amount excreted in the urine. Using the clearance formula: $C = \frac{U \times V}{P}$ *(Where U = Urine concentration, V = Urine flow rate, P = Plasma concentration)* $C = \frac{25 \text{ mg/ml} \times 10 \text{ ml/min}}{2 \text{ mg/ml}} = \mathbf{125 \text{ ml/min}}$ Since Inulin handles only filtration, its clearance rate ($125 \text{ ml/min}$) is exactly equal to the GFR. **2. Why Other Options are Wrong:** * **Option B:** Clearance is greater than GFR only for substances that undergo **tubular secretion** (e.g., PAH, Creatinine). * **Option C:** Clearance is less than GFR for substances that undergo **tubular reabsorption** (e.g., Glucose, Urea, Sodium). * **Option D:** GFR can be easily calculated using the standard clearance formula provided the plasma and urine concentrations of a marker like Inulin are known. **Clinical Pearls for NEET-PG:** * **Creatinine Clearance:** In clinical practice, endogenous creatinine is used to estimate GFR. However, it **slightly overestimates GFR** because a small amount of creatinine is secreted by the tubules. * **PAH (Para-aminohippurate):** Used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and almost completely secreted. * **Filtration Fraction (FF):** Calculated as $GFR / RPF$. Normal value is approximately **20%**.

Q: Which substance is not significantly absorbed in the loop of Henle?

Urea. **Explanation:** The Loop of Henle (LoH) plays a critical role in the countercurrent multiplier system, primarily focusing on the reabsorption of electrolytes to create a hypertonic medullary interstitium. **Why Urea is the correct answer:** Urea is unique because it is primarily **secreted** into the thin descending limb of the Loop of Henle rather than being absorbed. While urea is heavily reabsorbed in the Proximal Convoluted Tubule (50%) and the Medullary Collecting Ducts (via UT-A1/A3 transporters under ADH influence), the Loop of Henle itself is not a site of significant urea absorption. Instead, urea "recycles" from the medullary interstitium back into the tubular lumen of the LoH to maintain the osmotic gradient. **Why the other options are incorrect:** * **Na+, Cl-, and K+:** These electrolytes are significantly reabsorbed in the **Thick Ascending Limb (TAL)** of the Loop of Henle. This occurs via the **NKCC2 transporter** (Sodium-Potassium-2 Chloride symporter). Approximately 25% of the filtered load of these ions is reclaimed here. This segment is known as the "diluting segment" because it is impermeable to water but highly active in solute reabsorption. **High-Yield Clinical Pearls for NEET-PG:** * **NKCC2 Transporter:** This is the target of **Loop Diuretics** (e.g., Furosemide). Inhibition leads to powerful diuresis and potential hypokalemia. * **Bartter Syndrome:** A genetic defect in the NKCC2 transporter (or related channels in the TAL) that mimics chronic loop diuretic use. * **Countercurrent Multiplier:** The TAL reabsorbs NaCl without water, while the Descending Limb is permeable to water but not solutes. This separation is essential for concentrating urine. * **Urea Recycling:** This process is essential for maintaining the high osmolarity of the inner medulla, which allows for water reabsorption in the collecting ducts.

Q: Hypertonic urine is excreted due to water absorption in which part of the nephron?

Collecting ducts. ### Explanation The final concentration of urine (hypertonicity) is primarily determined in the **collecting ducts** under the influence of **Antidiuretic Hormone (ADH/Vasopressin)**. **1. Why Option A is Correct:** While water is reabsorbed in various segments of the nephron, the collecting duct is the **final site** where the urine concentration is adjusted. In the presence of ADH, **Aquaporin-2 channels** are inserted into the apical membrane of the collecting duct cells. This allows water to move out of the tubule into the highly medullary interstitium via osmosis, resulting in the excretion of concentrated (hypertonic) urine. **2. Why the Other Options are Incorrect:** * **Distal Convoluted Tubule (DCT):** The early DCT is part of the "diluting segment." It is relatively impermeable to water but actively reabsorbs solutes, making the tubular fluid more dilute (hypotonic), not hypertonic. * **Ascending Limb of the Loop of Henle:** This segment is **impermeable to water** but actively transports NaCl into the interstitium (via the NKCC2 transporter). It is known as the "diluting segment" because it removes solute without water, decreasing tubular tonicity. * **Descending Limb of the Loop of Henle:** While water is reabsorbed here, this segment contributes to the **Countercurrent Multiplier** system to create a medullary gradient. It does not determine the final tonicity of the excreted urine. **Clinical Pearls for NEET-PG:** * **V2 Receptors:** ADH acts on V2 receptors in the collecting duct to increase cAMP, leading to Aquaporin-2 insertion. * **Obligatory Water Reabsorption:** Occurs in the PCT (65%) and is independent of ADH. * **Facultative Water Reabsorption:** Occurs in the collecting ducts and is **ADH-dependent**. * **Diabetes Insipidus:** A deficiency of ADH (Central) or resistance to it (Nephrogenic) prevents water reabsorption in the collecting ducts, leading to large volumes of dilute (hypotonic) urine.

Q: Which of the following substances is NOT significantly absorbed in the proximal convoluted tubule (PCT)?

H+. The **Proximal Convoluted Tubule (PCT)** is the "workhorse" of the nephron, responsible for the bulk reabsorption of essential solutes. ### **Why H+ is the Correct Answer** Hydrogen ions ($H^+$) are **secreted**, not absorbed, in the PCT. This occurs primarily via the **$Na^+$-$H^+$ Exchanger 3 (NHE3)** on the apical membrane. This secretion is a vital step in acid-base balance, as the secreted $H^+$ reacts with filtered bicarbonate ($HCO_3^-$) to facilitate its indirect reabsorption. Therefore, $H^+$ moves from the peritubular capillaries/tubular cells into the lumen. ### **Why the Other Options are Incorrect** * **Bicarbonate ($HCO_3^-$):** Approximately **80-90%** of filtered bicarbonate is reabsorbed in the PCT. This is mediated by carbonic anhydrase (CA), which converts luminal $HCO_3^-$ to $CO_2$ and $H_2O$ for cellular entry. * **Sodium ($Na^+$):** About **65%** of filtered sodium is reabsorbed here via various symporters (with glucose/amino acids) and antiporters. It is the primary driving force for the reabsorption of water and other solutes. * **Phosphate ($PO_4^{3-}$):** Roughly **80%** of filtered phosphate is reabsorbed in the PCT via $Na^+$-$PO_4^{3-}$ cotransporters (NaPi-IIa). This process is clinically significant as it is inhibited by **Parathyroid Hormone (PTH)**. ### **High-Yield NEET-PG Pearls** * **Obligatory Water Reabsorption:** 65% of water is reabsorbed in the PCT iso-osmotically. * **Glucose & Amino Acids:** 100% are reabsorbed in the early PCT (S1 segment). * **Carbonic Anhydrase Inhibitors (Acetazolamide):** Act specifically on the PCT, leading to bicarbonate loss and metabolic acidosis. * **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, urate, and phosphate in the urine.

Question 1

Which of the following substances undergoes the least reabsorption through the renal tubule?

Accepted Answer

Urea

Answer

Glucose

Answer

HCO3

Answer

Uric acid

Question 2

What is the renal threshold for glycosuria?

Accepted Answer

180 mg/dL

Answer

100 mg/dL

Answer

300 mg/dL

Answer

350 mg/dL

Question 3

Which prostaglandin causes renal vasodilation?

Accepted Answer

PGE2

Answer

PGD2

Answer

PGF2

Answer

All of the above

Question 4

Which of the following characteristics should an ideal substance for measuring Glomerular Filtration Rate (GFR) NOT possess?

Accepted Answer

Should be secreted in tubules

Answer

Should be non-toxic

Answer

Should not remain in the body

Answer

Should not be protein bound

Question 5

Given a urine flow rate of 10 ml/min, plasma inulin of 2 mg/ml, and urine inulin of 25 mg/ml, which of the following statements is true?

Accepted Answer

Inulin clearance equals Glomerular Filtration Rate (GFR).

Answer

Inulin clearance is greater than GFR.

Answer

Inulin clearance is less than GFR.

Answer

GFR cannot be calculated with the given data.

Question 6

All of the following statements about the control of micturition are true except?

Accepted Answer

An individual with destruction of the sacral dorsal roots can still have a micturition reflex.

Answer

An individual with a spinal cord injury at L1 can still have a micturition reflex.

Answer

The micturition reflex can occur without voiding any urine.

Answer

Bladder volume can be more than double that present when the first urge to void occurs.

Question 7

Which substance is not significantly absorbed in the loop of Henle?

Accepted Answer

Urea

Answer

K+

Answer

Cl-

Answer

Na+

Question 8

Hypertonic urine is excreted due to water absorption in which part of the nephron?

Accepted Answer

Collecting ducts

Answer

Distal convoluted tubule (DCT)

Answer

Ascending limb of the loop of Henle

Answer

Descending limb of the loop of Henle

Question 9

Which of the following statements regarding nephron function is true?

Accepted Answer

The osmolality of intratubular content in the proximal convoluted tubule (PCT) is isotonic to the surrounding interstitium.

Answer

The ascending thick limb is permeable to water.

Answer

The descending thin limb is impermeable to water.

Answer

The osmolality of intra-tubular content in the distal convoluted tubule (DCT) is greater than the surrounding interstitium.

Question 10

Which of the following substances is NOT significantly absorbed in the proximal convoluted tubule (PCT)?

Accepted Answer

H+

Answer

Bicarbonate

Answer

Sodium

Answer

Phosphate

Renal Physiology — MCQs

Renal Physiology — MCQs

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