Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 391: A patient has a blood glucose level of 200 mg/dL and a GFR of 90 mL/min. The transport maximum for glucose is 150 mg/min. What is the amount of glucose excreted per minute?

A. 30 mg/min (Correct Answer)
B. 40 mg/min
C. 20 mg/min
D. 50 mg/min

Explanation: To solve this problem, we must apply the fundamental physiological principle of renal glucose handling: **Excretion = Filtered Load – Reabsorption.** ### 1. Calculation of the Correct Answer (A) * **Filtered Load:** This is the amount of glucose filtered through the glomerulus per minute. * *Formula:* Filtered Load = Plasma Glucose Concentration × GFR * *Calculation:* 200 mg/dL (which is 2 mg/mL) × 90 mL/min = **180 mg/min**. * **Reabsorption:** Glucose is reabsorbed in the proximal convoluted tubule via SGLT transporters. However, this process has a limit known as the **Transport Maximum ($T_m$)**. * In this patient, the $T_m$ is **150 mg/min**. Since the filtered load (180 mg/min) exceeds the $T_m$, the kidney can only reabsorb 150 mg/min. * **Excretion:** * *Calculation:* 180 mg/min (Filtered) – 150 mg/min (Reabsorbed) = **30 mg/min**. ### 2. Why Other Options are Incorrect * **B (40 mg/min):** This would occur if the filtered load was 190 mg/min or the $T_m$ was 140 mg/min. * **C (20 mg/min):** This would occur if the filtered load was 170 mg/min. * **D (50 mg/min):** This would occur if the filtered load was 200 mg/min. ### 3. Clinical Pearls for NEET-PG * **Renal Threshold:** The plasma glucose concentration at which glucose first appears in the urine (glycosuria) is typically **180 mg/dL**. * **Splay:** The curve of glucose excretion is not a sharp angle; it is rounded. This "splay" occurs because not all nephrons have the same $T_m$ and due to the low affinity of transporters near saturation. * **Transporters:** Glucose is reabsorbed by **SGLT-2** (90%, early PCT) and **SGLT-1** (10%, late PCT). SGLT-2 inhibitors (e.g., Dapagliflozin) are now key drugs in managing Diabetes Mellitus.

Question 392: Maximum reabsorption of sodium takes place in which part of the nephron?

A. Proximal convoluted tubule (Correct Answer)
B. Distal convoluted tubule
C. Loop of Henle
D. Collecting duct

Explanation: **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for the reabsorption of the glomerular filtrate. Approximately **65-70%** of filtered sodium (Na+) and water is reabsorbed here. This process is driven by the Na+/K+ ATPase pump on the basolateral membrane, which creates a gradient for sodium to enter the cell from the lumen via various symporters (e.g., Na-Glucose, Na-Amino acid) and antiporters (Na-H exchanger). **Analysis of Options:** * **Proximal Convoluted Tubule (Correct):** It possesses a "brush border" of microvilli that significantly increases the surface area for massive, iso-osmotic reabsorption of solutes and water. * **Loop of Henle:** The thick ascending limb reabsorbs about **20-25%** of filtered sodium via the Na-K-2Cl cotransporter (NKCC2). It is crucial for the countercurrent multiplier system but reabsorbs less than the PCT. * **Distal Convoluted Tubule:** This segment reabsorbs only about **5%** of sodium via the Na-Cl symporter. It is the site of action for Thiazide diuretics. * **Collecting Duct:** This is the "fine-tuning" segment, reabsorbing only **1-3%** of sodium. Sodium reabsorption here is regulated by **Aldosterone** via ENaC channels. **High-Yield Clinical Pearls for NEET-PG:** * **Obligatory Reabsorption:** Reabsorption in the PCT is "obligatory" (independent of hormones), whereas reabsorption in the collecting duct is "facultative" (hormone-dependent). * **Glucose & Amino Acids:** 100% of filtered glucose and amino acids are reabsorbed in the PCT (specifically the S1 segment). * **Carbonic Anhydrase:** The PCT is the site of action for Acetazolamide, which inhibits sodium bicarbonate reabsorption.

Question 393: Acute renal failure most commonly results in which of the following electrolyte and acid-base disturbances?

A. Hyperkalemia with metabolic alkalosis
B. Hypokalemia with metabolic alkalosis
C. Hyperkalemia with metabolic acidosis (Correct Answer)
D. Hypokalemia with metabolic acidosis

Explanation: **Explanation:** In Acute Renal Failure (ARF), now commonly termed Acute Kidney Injury (AKI), the sudden decline in GFR leads to the accumulation of metabolic waste products and the failure of homeostatic mechanisms. **Why Hyperkalemia and Metabolic Acidosis occur:** 1. **Hyperkalemia:** The kidneys are the primary route for potassium excretion. In ARF, the reduced distal delivery of sodium and decreased GFR impair the secretion of $K^+$ in the collecting ducts. Additionally, the associated acidosis causes an intracellular-to-extracellular shift of $K^+$ (as $H^+$ ions enter cells, $K^+$ exits to maintain electroneutrality). 2. **Metabolic Acidosis:** The kidneys fail to excrete "fixed" non-volatile acids (like phosphates and sulfates) produced by daily metabolism. Furthermore, there is a failure to reabsorb filtered $HCO_3^-$ and a defect in ammonia production ($NH_3$), which is essential for buffering $H^+$ ions in the urine. This results in a **High Anion Gap Metabolic Acidosis (HAGMA)**. **Analysis of Incorrect Options:** * **Options A & B (Metabolic Alkalosis):** Alkalosis is rare in ARF. It typically occurs with massive vomiting or excessive diuretic use, whereas renal failure inherently causes acid retention. * **Options B & D (Hypokalemia):** Hypokalemia is not a feature of the maintenance phase of ARF. It is more commonly seen in the **recovery (diuretic) phase** of AKI or in specific conditions like Renal Tubular Acidosis (RTA). **NEET-PG High-Yield Pearls:** * **ECG in Hyperkalemia:** Look for tall "tented" T-waves, widened QRS, and loss of P-waves. * **Exception:** In AKI caused by aminoglycosides or cisplatin, patients may occasionally present with hypokalemia due to tubular wasting. * **Anion Gap:** AKI typically causes a High Anion Gap Metabolic Acidosis due to the retention of unmeasured anions (sulfates, phosphates).

Question 394: Renin is secreted by which cells?

A. Juxtaglomerular cells
B. Cells in macula densa
C. Granular cells (Correct Answer)
D. Mesangial cells

Explanation: **Explanation:** **1. Why "Granular cells" is correct:** Renin is synthesized, stored, and secreted by the **Granular cells** (also known as Juxtaglomerular cells). These are specialized **modified smooth muscle cells** located primarily in the tunica media of the **afferent arteriole** at the point where it enters the glomerulus. They act as intrarenal baroreceptors, sensing changes in renal perfusion pressure and releasing renin in response to hypotension or sympathetic stimulation. **2. Analysis of Incorrect Options:** * **Juxtaglomerular (JG) cells:** While "JG cells" is often used synonymously with Granular cells in many textbooks, in the context of a highly specific NEET-PG question where both are listed, **Granular cells** is the more precise histological term for the renin-secreting units. (Note: If "Granular cells" were absent, JG cells would be the correct choice). * **Macula densa:** These are specialized columnar epithelial cells in the **Distal Convoluted Tubule (DCT)**. They act as **chemoreceptors** that sense sodium chloride (NaCl) concentration in the tubular fluid; they do not secrete renin but signal the granular cells to do so. * **Mesangial cells:** These provide structural support to the glomerular capillaries and have contractile properties. They are not involved in the primary secretion of renin. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Juxtaglomerular Apparatus (JGA)** consists of three components: Granular cells, Macula densa, and Lacis cells (Extraglomerular mesangial cells). * **Stimuli for Renin Release:** 1. Decreased renal perfusion pressure (Baroreceptor mechanism), 2. Decreased NaCl delivery to macula densa, 3. Increased Sympathetic activity (via **$\beta_1$ receptors**). * **Rate-limiting step:** Renin release is the rate-limiting step of the Renin-Angiotensin-Aldosterone System (RAAS).

Question 395: The Water Deprivation Test is used to assess which of the following aspects of renal function?

A. Glomerular Function
B. Proximal Tubular Function
C. Distal Tubular Function (Correct Answer)
D. Renal Plasma Flow

Explanation: ### Explanation **1. Why Distal Tubular Function is Correct:** The Water Deprivation Test (also known as the Miller-Moses test) is the gold standard for evaluating the kidney's **concentrating ability**. This process is primarily mediated by the **Distal Convoluted Tubule (DCT) and Collecting Ducts**. Under conditions of water deprivation, plasma osmolality rises, triggering the release of **Antidiuretic Hormone (ADH)** from the posterior pituitary. ADH acts on the V2 receptors in the distal segments of the nephron to insert aquaporin-2 channels, allowing water reabsorption. A normal response is the production of highly concentrated urine. If the distal tubules fail to respond to ADH (Nephrogenic Diabetes Insipidus) or if ADH is absent (Central Diabetes Insipidus), the urine remains dilute despite dehydration. **2. Why Other Options are Incorrect:** * **A. Glomerular Function:** This is assessed by the **Glomerular Filtration Rate (GFR)** using clearance studies (e.g., Inulin or Creatinine clearance). * **B. Proximal Tubular Function:** This is evaluated by measuring the reabsorption of substances like glucose (TmG), amino acids, or bicarbonate, often impaired in Fanconi Syndrome. * **D. Renal Plasma Flow (RPF):** This is measured using the clearance of **Para-aminohippuric acid (PAH)**, as it is both filtered and secreted. **3. Clinical Pearls for NEET-PG:** * **Primary Polydipsia vs. DI:** The test helps differentiate psychogenic polydipsia from Diabetes Insipidus (DI). * **Desmopressin Challenge:** If urine osmolality does not rise after water deprivation, **Exogenous Vasopressin (Desmopressin)** is administered. * *Rise in Osmolality (>50%):* Central DI. * *No/Minimal Rise (<10%):* Nephrogenic DI. * **Normal Response:** A healthy individual should achieve a urine osmolality of >800 mOsm/kg during the test.

Question 396: Positively charged particles can be more easily filtered than negatively charged particles because?

A. Positively charged particles are of smaller size
B. The presence of positively charged particles on the filtering membrane attracts positively charged particles.
C. The presence of negatively charged sialoproteins on the filtering membrane repels negatively charged particles and allows positively charged particles. (Correct Answer)
D. None of the above

Explanation: ### Explanation The filtration of substances across the glomerular filtration barrier is determined by two primary factors: **molecular size** and **electrical charge**. **1. Why Option C is Correct:** The glomerular capillary wall (specifically the basement membrane and the podocyte glycocalyx) is lined with **negatively charged sialoproteins** (such as podocalyxin) and heparan sulfate proteoglycans. According to Coulomb’s Law, like charges repel. Therefore, these negatively charged glycoproteins create an electrostatic barrier that repels anions (negatively charged particles like albumin) and facilitates the passage of cations (positively charged particles). This is known as **charge selectivity**. **2. Why Other Options are Incorrect:** * **Option A:** Charge and size are independent properties. A positively charged particle is not inherently smaller than a negatively charged one. While smaller particles filter more easily than larger ones (**size selectivity**), the question specifically asks about the influence of charge. * **Option B:** This is factually incorrect. The filtering membrane is negatively charged, not positively charged. If the membrane were positive, it would repel cations. **Clinical Pearls for NEET-PG:** * **Minimal Change Disease (MCD):** The primary pathology is the loss of the negative charge on the glomerular basement membrane (due to T-cell mediated cytokine injury). This results in massive proteinuria (specifically albuminuria) because the charge barrier is lost, even though the size barrier remains intact. * **Albumin:** Despite having a molecular radius (~3.6 nm) slightly smaller than the pore size of the slit diaphragm, albumin is restricted from filtration primarily due to its strong negative charge. * **Selectivity Index:** A low index (<0.1) indicates highly selective proteinuria (common in MCD), whereas a high index (>0.2) suggests non-selective proteinuria (seen in structural damage like FSGS).

Question 397: Calculate the filtration fraction if the Glomerular Filtration Rate (GFR) is 125 ml/min and the Renal Plasma Flow is 625 ml/min.

A. 5%
B. 20% (Correct Answer)
C. 50%
D. 10%

Explanation: **Explanation** **1. Understanding the Correct Answer (B: 20%)** The **Filtration Fraction (FF)** represents the proportion of the renal plasma flow that is actually filtered across the glomerular capillaries into the Bowman’s space. It is a critical indicator of renal efficiency and hemodynamics. The formula to calculate Filtration Fraction is: $$\text{FF} = \frac{\text{GFR}}{\text{RPF}} \times 100$$ Plugging in the values from the question: $$\text{FF} = \frac{125 \text{ ml/min}}{625 \text{ ml/min}} = 0.20 \text{ or } 20\%$$ This means that 20% of the plasma entering the kidneys is filtered, while the remaining 80% leaves the glomerulus via the efferent arteriole to become peritubular capillary flow. **2. Analysis of Incorrect Options** * **A (5%) & D (10%):** These values are too low for a healthy adult. Such low fractions may be seen in conditions where GFR is severely compromised relative to blood flow, such as acute glomerulonephritis. * **C (50%):** This is physiologically improbable. A filtration fraction this high would excessively increase the oncotic pressure in the peritubular capillaries, potentially leading to sluggish blood flow and impaired renal function. **3. NEET-PG High-Yield Pearls** * **Normal Range:** The physiological FF is typically **19–20%**. * **Effect of Efferent Vasoconstriction:** Angiotensin II preferentially constricts the efferent arteriole. This increases glomerular hydrostatic pressure, thereby **increasing the FF** (even if RPF decreases). * **Clinical Significance:** An increased FF is often seen in **Congestive Heart Failure (CHF)** because the body attempts to maintain GFR despite a drop in total renal blood flow. * **RPF vs. RBF:** Remember that RPF is only the plasma component. If the question provides Renal Blood Flow (RBF), you must first calculate RPF using the formula: $RPF = RBF \times (1 - \text{Hematocrit})$.

Question 398: If urine dipstick shows +3 for protein, what is the approximate protein level in mg/dl?

A. 30 mg/dl
B. 50 mg/dl
C. 100 mg/dl
D. 300 mg/dl (Correct Answer)

Explanation: **Explanation:** The urine dipstick test is a semi-quantitative method primarily used to detect **Albuminuria**. It utilizes the "protein error of indicators" principle (usually tetrabromphenol blue), where the color change of the reagent strip corresponds to the concentration of protein in the urine. **Why Option D is correct:** The standard grading for protein on a dipstick is as follows: * **Trace:** 15–30 mg/dl * **1+:** 30–100 mg/dl (Average ~30 mg/dl) * **2+:** 100–300 mg/dl (Average ~100 mg/dl) * **3+:** 300–1000 mg/dl (**Average ~300 mg/dl**) * **4+:** >1000 mg/dl (Average ~1000 mg/dl or more) Therefore, a **+3** reading correlates specifically to an approximate level of **300 mg/dl**. **Analysis of Incorrect Options:** * **Option A (30 mg/dl):** This corresponds to a **1+** reading. * **Option B (50 mg/dl):** This falls within the range of a 1+ reading but is not the standard value for 3+. * **Option C (100 mg/dl):** This corresponds to a **2+** reading. **High-Yield Clinical Pearls for NEET-PG:** 1. **Selectivity:** The dipstick is highly sensitive to **Albumin** but insensitive to globulins, Bence-Jones proteins (light chains), or hemoglobin. 2. **False Positives:** Highly alkaline urine (pH > 8.0), concentrated urine, or contamination with chlorhexidine/quaternary ammonium compounds. 3. **False Negatives:** Very dilute urine or presence of non-albumin proteins (e.g., Multiple Myeloma). 4. **Confirmatory Test:** The Sulfosalicylic Acid (SSA) precipitation test is used to detect all types of proteins, including globulins.

Question 399: Administration of two litres of normal saline over 4 hours will stimulate the secretion of which of the following?

A. Atrial natriuretic peptide (Correct Answer)
B. Interleukin-2
C. Tumor Necrosis Factor-alpha
D. Prostaglandins

Explanation: **Explanation:** The administration of two litres of normal saline (an isotonic solution) leads to **volume expansion** of the extracellular fluid (ECF) compartment. **Why Atrial Natriuretic Peptide (ANP) is correct:** When ECF volume increases, there is a corresponding increase in venous return to the heart. This causes **stretching of the atrial myocytes**. In response to this mechanical stretch, the atria secrete ANP. ANP acts as a potent hypotensive agent by promoting **natriuresis** (sodium excretion) and **diuresis** (water excretion) through the dilation of afferent arterioles and constriction of efferent arterioles in the kidney. It also inhibits the Renin-Angiotensin-Aldosterone System (RAAS), helping the body return to a normovolemic state. **Why the other options are incorrect:** * **Interleukin-2 (IL-2) and Tumor Necrosis Factor-alpha (TNF-α):** These are inflammatory cytokines. Their secretion is triggered by immune responses, infections, or tissue injury, not by simple volume expansion. * **Prostaglandins:** While renal prostaglandins (like PGE2) play a role in maintaining renal blood flow, they are typically synthesized in response to vasoconstrictors (like Angiotensin II) to prevent excessive ischemia, rather than as a primary response to volume loading. **Clinical Pearls for NEET-PG:** * **Brain Natriuretic Peptide (BNP):** Similar to ANP but primarily secreted by the **ventricles** in response to pressure overload or volume expansion. It is a key clinical marker for heart failure. * **Isotonic Saline Effect:** Infusing 0.9% NaCl increases ECF volume but does not change intracellular fluid (ICF) volume because there is no osmotic gradient created. * **ANP Mechanism:** It works via the **cGMP** second messenger system (guanylyl cyclase).

Question 400: What percentage of renal plasma flow is represented by the Glomerular Filtration Rate (GFR)?

A. 20% (Correct Answer)
B. 38%
C. 50%
D. 60%

Explanation: ### Explanation The correct answer is **A. 20%**. The relationship between the Glomerular Filtration Rate (GFR) and the Renal Plasma Flow (RPF) is defined by the **Filtration Fraction (FF)**. The formula is: $$\text{Filtration Fraction} = \frac{\text{GFR}}{\text{RPF}}$$ In a healthy adult: * **Average GFR:** ~125 mL/min * **Average RPF:** ~600–650 mL/min * **Calculation:** $125 / 625 = 0.20$ or **20%**. This means that as blood passes through the glomerular capillaries, approximately one-fifth of the plasma is filtered into Bowman’s space, while the remaining 80% leaves the glomerulus via the efferent arteriole to become peritubular capillary flow. **Analysis of Incorrect Options:** * **B (38%):** This value is physiologically too high for a resting state. Such a high fraction would excessively increase the oncotic pressure in the peritubular capillaries, potentially disrupting tubular reabsorption dynamics. * **C & D (50% & 60%):** These values are incorrect. If 50-60% of plasma were filtered, the remaining blood in the efferent arteriole would become extremely viscous (due to concentrated red cells and proteins), severely impairing renal microcirculation. **Clinical Pearls for NEET-PG:** 1. **Filtration Fraction Dynamics:** FF **increases** in states of efferent arteriolar constriction (e.g., low-dose Angiotensin II) because GFR is maintained while RPF decreases. 2. **Renal Blood Flow (RBF) vs. RPF:** Remember that RBF (~1100 mL/min) includes the volume of red blood cells, whereas GFR is only a fraction of the **plasma** flow. 3. **Measurement:** GFR is best measured by **Inulin clearance** (gold standard), while RPF is measured by **PAH (Para-aminohippuric acid) clearance**.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

Tubular Reabsorption and Secretion

Practice Questions

Concentration and Dilution of Urine

Practice Questions

Acid-Base Regulation by the Kidneys

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Sodium and Water Balance

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Potassium Regulation

Practice Questions

Calcium and Phosphate Handling

Practice Questions

Micturition Physiology

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Renal Function Tests

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Integrative Responses to Fluid Challenges

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