Renal Physiology Practice Questions

Q: What is the clearance of a substance if its concentration in plasma is 10 mg%, concentration in urine is 100 mg%, and urine flow is 2 ml/min?

20 ml/min. ***20 ml/min*** - Renal clearance ($C$) is calculated using the formula **$C = (U \times V) / P$**, where $U$ is urine concentration, $V$ is urine flow rate, and $P$ is plasma concentration. - By substituting the given values: **$(100\, mg\% \times 2\, ml/min) / 10\, mg\% = 20\, ml/min$**, which represents the volume of plasma cleared of the substance per minute. *0.02 ml/min* - This value is significantly lower than the calculated result and likely stems from a **calculation error** involving incorrect decimal placement. - It does not follow the standard **clearance equation** logic based on the provided concentrations and flow rate. *0.2 ml/min* - This incorrect value could result from a **calculation error** such as dividing $P$ by $U$ instead of $U$ by $P$. - It fails to account for the fact that the substance is **concentrated 10-fold** in the urine compared to the plasma. *2 ml/min* - This result would be obtained if the **U/P ratio** was 1, meaning the urine and plasma concentrations were identical. - Since the urine concentration is **10 times higher** than the plasma concentration, the clearance must be higher than the urine flow rate itself.

Q: Rapid diffusion of water across cell membranes depends on the presence of water channels, called aquaporins. Which aquaporin is present in the proximal convoluted tubule?

Aquaporin 1. **Explanation:** The correct answer is **Aquaporin 1 (AQP1)**. In the kidney, approximately 65% of filtered water is reabsorbed in the **Proximal Convoluted Tubule (PCT)**. This rapid, constitutive movement of water occurs via AQP1 channels located on both the apical and basolateral membranes, as well as the descending limb of the Loop of Henle. Unlike other segments, AQP1 in the PCT is **not regulated by ADH** (Vasopressin), ensuring constant water reabsorption coupled with solute transport. **Analysis of Incorrect Options:** * **Aquaporin 2 (AQP2):** Found exclusively in the **principal cells** of the collecting ducts. It is the only aquaporin regulated by **ADH**. ADH binding to V2 receptors causes the insertion of AQP2 into the apical membrane. * **Aquaporin 5 (AQP5):** Primarily located in **secretory glands** (salivary, lacrimal, and sweat glands) and alveolar type I cells in the lungs. It is not a major renal aquaporin. * **Aquaporin 9 (AQP9):** Functions as an "aquaglyceroporin" (transporting water and glycerol) and is mainly expressed in the **liver**, leukocytes, and brain. **High-Yield NEET-PG Pearls:** * **AQP1:** PCT and Thin Descending Limb (responsible for "obligatory" water reabsorption). * **AQP2:** Apical membrane of Collecting Duct (target of ADH; deficiency causes **Nephrogenic Diabetes Insipidus**). * **AQP3 & AQP4:** Basolateral membrane of Collecting Duct (provide the exit pathway for water into the interstitium). * **Mnemonic:** "AQP**1** is **First** (PCT), AQP**2** is **Two**-wards the end (Collecting Duct)."

Q: If a substance "X" has more renal clearance than the Glomerular Filtration Rate (GFR), what does it imply about its handling by the renal tubules?

The substance "X" is being secreted by the renal tubules.. **Explanation:** The concept of **Renal Clearance ($C_x$)** refers to the volume of plasma cleared of a substance per unit time. To understand how the kidney handles a substance, we compare its clearance to the **Glomerular Filtration Rate (GFR)**, typically measured using Inulin. 1. **Why Option B is Correct:** If $C_x > GFR$, it means that the amount of substance "X" appearing in the urine is greater than the amount that was filtered at the glomerulus. This "extra" amount must have been added to the tubular fluid via **tubular secretion** from the peritubular capillaries. A classic example is Para-aminohippuric acid (PAH). 2. **Why Other Options are Incorrect:** * **Option A:** If a substance is primarily reabsorbed (e.g., Glucose, Urea), its clearance will be **less than the GFR** ($C_x GFR$ specifically indicates that **secretion exceeds reabsorption**. * **Option D:** If a substance is neither secreted nor reabsorbed (e.g., Inulin), its clearance is **exactly equal to the GFR** ($C_x = GFR$). **High-Yield Clinical Pearls for NEET-PG:** * **Inulin Clearance:** The gold standard for measuring GFR ($C_{Inulin} = GFR$). * **Creatinine Clearance:** Slightly overestimates GFR because it undergoes a small amount of tubular secretion ($C_{Cr} \approx 1.1 \times GFR$). * **PAH Clearance:** Used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and almost completely secreted. * **Glucose Clearance:** Normally zero because it is 100% reabsorbed in the proximal tubule (up to its transport maximum, $T_m$).

Q: Which form of angiotensin is physiologically inactive?

Angiotensin I. **Explanation:** The Renin-Angiotensin-Aldosterone System (RAAS) is a hormonal cascade critical for blood pressure regulation. The correct answer is **Angiotensin I** because it serves primarily as a precursor molecule with no significant direct biological activity. **1. Why Angiotensin I is the correct answer:** Angiotensin I (a decapeptide) is produced when **Renin** acts on Angiotensinogen. While it is the first product in the cascade, it is **physiologically inactive**. It must be converted into Angiotensin II by the **Angiotensin-Converting Enzyme (ACE)**, primarily in the pulmonary capillaries, to exert any systemic effects. **2. Why the other options are incorrect:** * **Angiotensin II:** The most potent biological effector of the RAAS. It causes powerful vasoconstriction, stimulates aldosterone release, and increases sympathetic activity. * **Angiotensin III:** Formed by the removal of an amino acid from Angiotensin II. It possesses about 40% of the pressor activity of Angiotensin II and is equally potent in stimulating **aldosterone secretion**. * **Angiotensin IV:** A hexapeptide metabolite. While less potent in blood pressure regulation, it has distinct physiological roles in the CNS, including memory enhancement and blood flow regulation. **NEET-PG High-Yield Pearls:** * **Rate-limiting step of RAAS:** The secretion of Renin from the Juxtaglomerular (JG) cells. * **ACE Inhibitors (ACEIs):** Drugs like Enalapril work by preventing the conversion of the inactive Angiotensin I to the active Angiotensin II. * **Site of ACE:** Primarily the luminal surface of vascular endothelial cells, especially in the **lungs**.

Q: Which of the following provides the most accurate measure of Glomerular Filtration Rate (GFR)?

Inulin clearance. ### Explanation **1. Why Inulin Clearance is the Correct Answer:** Inulin is a fructose polymer that serves as the **gold standard** for measuring GFR because it possesses the ideal characteristics of a glomerular marker. For a substance's clearance to equal GFR, it must be: * **Freely filtered** at the glomerulus. * **Neither reabsorbed nor secreted** by the renal tubules. * **Not metabolized or synthesized** by the kidney. * **Physiologically inert** and non-toxic. Since the amount of inulin filtered equals the amount excreted, the formula $Clearance = \frac{U \times V}{P}$ provides an exact measurement of GFR. **2. Why the Other Options are Incorrect:** * **A. Blood Urea Nitrogen (BUN):** This is an unreliable marker because urea is significantly reabsorbed (about 50%) by the tubules. Levels are also influenced by non-renal factors like high-protein diet, dehydration, and GI bleeds. * **B. Endogenous Creatinine Clearance:** While commonly used in clinical practice, it **overestimates GFR by 10–20%** because creatinine is not only filtered but also slightly secreted by the proximal tubules. * **D. PAH Clearance:** PAH is both filtered and extensively secreted. Its clearance is used to measure **Effective Renal Plasma Flow (eRPF)**, not GFR. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Creatinine vs. Inulin:** In clinical settings, creatinine is preferred because inulin requires continuous intravenous infusion and is expensive. * **Filtration Fraction (FF):** Calculated as $GFR / RPF$. Normal value is approximately **20%**. * **Cystatin C:** An emerging endogenous marker that is not affected by muscle mass or diet, making it more accurate than creatinine in certain populations. * **PAH Fact:** At low plasma concentrations, PAH clearance equals eRPF because its extraction ratio is nearly 90%.

Q: α - intercalated cells are present in which part of the nephron and what is their primary function?

Collecting duct, H+ secretion. **Explanation:** The correct answer is **C. Collecting duct, H+ secretion.** **1. Understanding the Mechanism (Why C is correct):** Intercalated cells are specialized cells found in the **Late Distal Tubule and Collecting Duct**. There are two types: Alpha (α) and Beta (β). * **α-intercalated cells** are responsible for **acid secretion**. They utilize a primary active transport mechanism, the **H+-ATPase pump** (and H+/K+-ATPase), on their apical membrane to pump H+ ions into the tubular lumen. Simultaneously, they reabsorb HCO3– into the blood via the Cl–/HCO3– exchanger (Basolateral side). This process is vital for blood pH regulation, especially during acidosis. **2. Analysis of Incorrect Options:** * **Option A & B:** While the late distal tubule contains these cells, the primary site of their physiological significance and highest density is the **Collecting Duct**. Furthermore, Option A incorrectly attributes HCO3– secretion to α-cells. * **Option D:** This describes the function of **β-intercalated cells**. β-cells are the "mirror image" of α-cells; they secrete HCO3– into the lumen and reabsorb H+ into the blood, which is essential during alkalosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Aldosterone Connection:** Aldosterone stimulates the H+-ATPase in α-intercalated cells. Therefore, primary hyperaldosteronism (Conn’s Syndrome) leads to metabolic alkalosis due to excessive H+ secretion. * **Distal RTA (Type 1):** This condition is caused by the failure of α-intercalated cells to secrete H+, leading to an inability to acidify urine (Urine pH > 5.5). * **Potassium Link:** α-intercalated cells reabsorb K+ via the H+/K+-ATPase pump; hence, chronic acidosis often leads to hyperkalemia.

Question 1

A patient with chronic heart failure shows elevated ADH levels despite low serum osmolality (270 mOsm/kg) and hyponatremia (125 mEq/L). Critically evaluating this paradox, which represents the most physiologically sound explanation?

Accepted Answer

Non-osmotic stimuli (baroreceptors sensing decreased effective volume) override osmotic inhibition

Answer

Osmoreceptor dysfunction causing inappropriate ADH secretion

Answer

Syndrome of inappropriate ADH secretion (SIADH) from cardiac medications

Answer

Primary polydipsia causing dilutional hyponatremia with secondary ADH elevation

Question 2

What is the clearance of a substance if its concentration in plasma is 10 mg%, concentration in urine is 100 mg%, and urine flow is 2 ml/min?

Accepted Answer

20 ml/min

Answer

0.02 ml/min

Answer

0.2 ml/min

Answer

2 ml/min

Question 3

Rapid diffusion of water across cell membranes depends on the presence of water channels, called aquaporins. Which aquaporin is present in the proximal convoluted tubule?

Accepted Answer

Aquaporin 1

Answer

Aquaporin 2

Answer

Aquaporin 5

Answer

Aquaporin 9

Question 4

Which protein in the glomerular basement membrane (GBM) is responsible for charge-dependent filtration?

Accepted Answer

Proteoglycan

Answer

Albumin

Answer

Collagen type IV

Answer

Fibronectin

Question 5

Which is the most sensitive index for renal tubular function?

Accepted Answer

Specific gravity of urine

Answer

Blood urea

Answer

Glomerular Filtration Rate (GFR)

Answer

Creatinine clearance

Question 6

What is the primary mechanism of renal autoregulation?

Accepted Answer

It minimizes the impact of changes in arterial blood pressure on renal sodium excretion.

Answer

It is associated with increased renal vascular resistance when arterial blood pressure is lowered from 100 to 80 mm Hg.

Answer

It mainly involves changes in the caliber of efferent arterioles.

Answer

It maintains a normal renal blood flow during severe hypotension (blood pressure, 50 mm Hg).

Question 7

If a substance "X" has more renal clearance than the Glomerular Filtration Rate (GFR), what does it imply about its handling by the renal tubules?

Accepted Answer

The substance "X" is being secreted by the renal tubules.

Answer

The substance "X" is primarily being reabsorbed by the renal tubules.

Answer

The substance "X" is undergoing both secretion and reabsorption by the renal tubules.

Answer

The substance "X" is neither secreted nor reabsorbed by the renal tubules.

Question 8

Which form of angiotensin is physiologically inactive?

Accepted Answer

Angiotensin I

Answer

Angiotensin II

Answer

Angiotensin III

Answer

Angiotensin IV

Question 9

Which of the following provides the most accurate measure of Glomerular Filtration Rate (GFR)?

Accepted Answer

Inulin clearance

Answer

Blood urea nitrogen (BUN)

Answer

Endogenous creatinine clearance

Answer

Para-aminohippuric acid (PAH) clearance

Question 10

α - intercalated cells are present in which part of the nephron and what is their primary function?

Accepted Answer

Collecting duct, H+ secretion

Answer

Distal convoluted tubule, HCO3– secretion

Answer

Distal convoluted tubule, H+ secretion

Answer

Collecting duct, HCO3– secretion

Renal Physiology — MCQs

Renal Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?