Renal Physiology Practice Questions

Q: In which segment of the nephron does Na+ reabsorption primarily occur via Na+-H+ exchange?

Proximal convoluted tubule (PCT). **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for sodium reabsorption, handling approximately 65% of the filtered load. In the early PCT, Na+ reabsorption is uniquely coupled with the secretion of H+ ions via the **NHE3 (Na+-H+ Exchanger)**. This process is driven by the sodium gradient created by the basolateral Na+-K+ ATPase and is fundamentally linked to **bicarbonate (HCO3-) reabsorption**. Carbonic anhydrase plays a crucial role here, making this segment the primary site for acid-base regulation. **Why other options are incorrect:** * **Loop of Henle:** In the Thick Ascending Limb (TAL), Na+ is primarily reabsorbed via the **NKCC2 symporter** (Na+-K+-2Cl-), which is the target of loop diuretics like furosemide. * **Distal Convoluted Tubule (DCT):** Na+ reabsorption here occurs mainly through the **NCC (Na+-Cl- cotransporter)**, which is sensitive to thiazide diuretics. * **Collecting Duct:** Na+ reabsorption in the Principal cells occurs via **ENaC (Epithelial Sodium Channels)**, regulated by aldosterone. **High-Yield Clinical Pearls for NEET-PG:** * **Carbonic Anhydrase Inhibitors (Acetazolamide):** These drugs act specifically on the PCT by inhibiting the Na+-H+ exchange mechanism indirectly, leading to alkaline urine. * **Angiotensin II:** Stimulates the NHE3 exchanger in the PCT, increasing Na+ and water reabsorption to maintain blood pressure. * **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, and bicarbonate in the urine.

Q: Para-aminohippuric acid (PAH) is used to measure which of the following?

Renal plasma flow. **Explanation:** **Para-aminohippuric acid (PAH)** is the gold standard for measuring **Effective Renal Plasma Flow (eRPF)**. This is because PAH is both freely filtered at the glomerulus and almost completely secreted by the proximal convoluted tubules. As a result, nearly all the PAH entering the renal artery is excreted into the urine in a single pass (extraction ratio ≈ 0.9). By calculating the clearance of PAH ($C_{PAH} = \frac{U_{PAH} \times V}{P_{PAH}}$), we can estimate the volume of plasma that flows through the kidneys per unit time. **Analysis of Incorrect Options:** * **A. Extracellular fluid volume:** Measured using substances that distribute throughout the ECF but do not enter cells, such as **Inulin**, **Mannitol**, or **Radioactive Sodium**. * **B. Glomerular filtration rate (GFR):** Measured using **Inulin** (the gold standard) or **Creatinine** (clinical marker). These substances are filtered but neither reabsorbed nor secreted. * **D. Plasma osmolarity:** This is a physical property of blood (normal: 280–295 mOsm/L) measured via osmometry or calculated using the formula: $2[Na^+] + \frac{Glucose}{18} + \frac{BUN}{2.8}$. **High-Yield Clinical Pearls for NEET-PG:** * **True Renal Plasma Flow:** Since extraction is not 100% (some blood bypasses secretory areas), True RPF = $\frac{eRPF}{0.9}$. * **Renal Blood Flow (RBF):** Calculated as $\frac{RPF}{1 - Hematocrit}$. * **Filtration Fraction (FF):** The ratio of GFR to RPF (Normal ≈ 20%). * **Transport Maximum ($T_m$):** PAH secretion is a carrier-mediated process and can be saturated. If plasma PAH levels exceed the $T_m$, clearance decreases and it no longer accurately reflects RPF.

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 of the following hormones is secreted by the kidney?

All of the above. The kidney is not only an excretory organ but also a vital endocrine organ that secretes several hormones essential for systemic homeostasis. **Explanation of the Correct Answer:** The correct answer is **D (All of the above)** because the kidney is the primary site of production for Renin, Erythropoietin, and the active form of Vitamin D. * **Renin:** Secreted by the **Juxtaglomerular (JG) cells** of the afferent arteriole. It is the rate-limiting enzyme of the Renin-Angiotensin-Aldosterone System (RAAS), which regulates blood pressure and fluid balance. * **1,25-dihydroxycholecalciferol (Calcitriol):** The kidney contains the enzyme **1-alpha-hydroxylase** (primarily in the proximal convoluted tubule), which converts inactive 25-hydroxyvitamin D into its active form, Calcitriol. This is crucial for calcium and phosphate absorption. * **Erythropoietin (EPO):** Produced by **interstitial cells in the peritubular capillary bed** (fibroblast-like cells) in response to hypoxia. It stimulates red blood cell production in the bone marrow. **Clinical Pearls for NEET-PG:** * **Chronic Kidney Disease (CKD):** Patients with CKD often present with **anemia** (due to EPO deficiency) and **renal osteodystrophy** (due to Vitamin D deficiency and secondary hyperparathyroidism). * **Thrombopoietin:** While primarily produced in the liver, the kidney also produces a small amount of thrombopoietin. * **Prostaglandins:** The kidney also synthesizes prostaglandins (PGE2 and PGI2), which act as local vasodilators to maintain renal blood flow.

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%**.

Question 1

If administration of exogenous vasopressin does not increase the osmolality of urine, what is the likely cause?

Accepted Answer

Renal hyposensitivity to Antidiuretic Hormone (ADH)

Answer

Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

Answer

Psychogenic polydipsia

Answer

Antidiuretic Hormone (ADH) deficiency

Question 2

In which segment of the nephron does Na+ reabsorption primarily occur via Na+-H+ exchange?

Accepted Answer

Proximal convoluted tubule (PCT)

Answer

Loop of Henle

Answer

Distal convoluted tubule (DCT)

Answer

Collecting duct

Question 3

Para-aminohippuric acid (PAH) is used to measure which of the following?

Accepted Answer

Renal plasma flow

Answer

Extracellular fluid volume

Answer

Glomerular filtration rate

Answer

Plasma osmolarity

Question 4

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

Accepted Answer

Urea

Answer

Glucose

Answer

HCO3

Answer

Uric acid

Question 5

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 6

Which of the following hormones is secreted by the kidney?

Accepted Answer

All of the above

Answer

Renin

Answer

1,25-dihydroxycholecalciferol

Answer

Erythropoietin

Question 7

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 8

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 9

Which of the following ions is NOT handled by the Loop of Henle?

Accepted Answer

Urea

Answer

Na+

Answer

K+

Answer

Cl-

Question 10

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.

Renal Physiology — MCQs

Renal Physiology — MCQs

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