Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 261: What is the maximum rate of glucose reabsorption in the renal tubules?

A. 375 mg/min (Correct Answer)
B. 150 mg/min
C. 180 mg/dL
D. 120 mg/min

Explanation: **Explanation:** The correct answer is **375 mg/min**. This value represents the **Transport Maximum ($T_m$)** for glucose in an average adult male. **1. Why 375 mg/min is correct:** Glucose is freely filtered at the glomerulus and reabsorbed in the **Proximal Convoluted Tubule (PCT)** via secondary active transport (SGLT-2 and SGLT-1). The $T_m$ is the maximum rate at which the renal tubules can reabsorb a substance. Once all the glucose transporters (SGLT) are saturated, any additional filtered glucose cannot be reabsorbed and is excreted in the urine. For males, this limit is typically **375 mg/min**, while for females, it is approximately **303 mg/min**. **2. Analysis of incorrect options:** * **180 mg/dL (Option C):** This is the **Renal Threshold** for glucose. It refers to the plasma concentration at which glucose first begins to appear in the urine. It is different from $T_m$ (which is a rate, mg/min). * **120 mg/min & 150 mg/min (Options B & D):** These values do not correspond to standard physiological constants for glucose transport. 125 mL/min is the average Glomerular Filtration Rate (GFR), which may cause confusion. **3. Clinical Pearls for NEET-PG:** * **Splay:** The curve of glucose excretion is not a sharp angle; it curves gradually. This "splay" occurs because not all nephrons have the same $T_m$ and because the affinity of transporters varies. * **SGLT-2 Inhibitors:** Drugs like Dapagliflozin work by lowering the $T_m$ for glucose, intentionally inducing glucosuria to treat Diabetes Mellitus. * **Site of Reabsorption:** 100% of glucose is normally reabsorbed in the PCT (90% by SGLT-2 in the early part, 10% by SGLT-1 in the later part).

Question 262: Intravenous infusion of 2.0 L of isotonic saline (0.9% NaCl) results in an increase in which of the following?

A. Intracellular fluid volume
B. Plasma aldosterone level
C. Plasma arginine vasopressin (AVP) concentration
D. Plasma atrial natriuretic peptide (ANP) concentration (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Plasma atrial natriuretic peptide (ANP) concentration** **Underlying Concept:** Isotonic saline (0.9% NaCl) has the same osmolarity as the extracellular fluid (ECF). When infused, it remains entirely within the **extracellular compartment**, leading to an expansion of the ECF volume and plasma volume. This volume expansion causes stretching of the atria, which triggers the release of **Atrial Natriuretic Peptide (ANP)**. ANP acts to promote natriuresis (sodium excretion) and diuresis to restore normal fluid balance. **Why Incorrect Options are Wrong:** * **A. Intracellular fluid (ICF) volume:** Since the saline is **isotonic**, there is no osmotic gradient created between the ECF and ICF. Therefore, no water moves into or out of the cells, and the ICF volume remains unchanged. * **B. Plasma aldosterone level:** Volume expansion is sensed by the juxtaglomerular apparatus, leading to a **decrease** in renin secretion. This results in a subsequent **decrease** in aldosterone levels to facilitate sodium excretion. * **C. Plasma arginine vasopressin (AVP) concentration:** AVP (ADH) is inhibited by two factors here: the increase in blood volume (sensed by baroreceptors) and the lack of increase in plasma osmolarity. Therefore, AVP levels would **decrease** to allow for water excretion. **High-Yield Facts for NEET-PG:** * **Isotonic Saline Distribution:** 100% remains in the ECF (approx. 1/4th in plasma, 3/4th in interstitial fluid). * **ANP Mechanism:** It increases GFR (dilates afferent/constricts efferent arterioles) and inhibits sodium reabsorption in the collecting ducts. * **Dextrose 5% (D5W):** Unlike saline, D5W acts as "free water" once glucose is metabolized, distributing across **both** ICF and ECF. * **Starling’s Law in the Atria:** Increased venous return → Increased atrial stretch → Increased ANP release.

Question 263: Glomerular filtration rate (GFR) is directly proportional to which of the following pressures?

A. Intracapillary glomerular pressure (Correct Answer)
B. Oncotic pressure
C. Bowman's capsular hydrostatic pressure
D. All of the above

Explanation: The Glomerular Filtration Rate (GFR) is determined by the **Starling forces** acting across the glomerular capillary membrane. The relationship is expressed by the formula: **GFR = Kf × [(Pgc – Pbc) – (πgc – πbc)]** ### 1. Why the Correct Answer is Right **Option A: Intracapillary glomerular pressure ($P_{gc}$)** This is the primary driving force for filtration. It is the hydrostatic pressure exerted by blood within the glomerular capillaries. Since it pushes fluid out of the capillary into the Bowman’s space, an increase in $P_{gc}$ leads to a direct increase in GFR. It is regulated by the resistance of the afferent and efferent arterioles. ### 2. Why the Other Options are Wrong * **Option B: Oncotic pressure ($\pi_{gc}$):** This is the osmotic pressure exerted by plasma proteins (like albumin). It acts as a "pulling" force that retains fluid inside the capillary. Therefore, GFR is **inversely proportional** to glomerular oncotic pressure; an increase in $\pi_{gc}$ decreases GFR. * **Option C: Bowman's capsular hydrostatic pressure ($P_{bc}$):** This is the pressure exerted by the fluid already present in the Bowman’s capsule. It opposes filtration. Therefore, GFR is **inversely proportional** to $P_{bc}$. Clinical conditions like kidney stones (urolithiasis) increase $P_{bc}$ and subsequently decrease GFR. ### 3. High-Yield Facts for NEET-PG * **Kf (Filtration Coefficient):** Represents the surface area and permeability. It is reduced in diseases like diabetes mellitus or chronic hypertension. * **Afferent Arteriole Constriction:** Decreases $P_{gc}$ and GFR. * **Efferent Arteriole Constriction:** Increases $P_{gc}$ and GFR (up to a point). * **Most sensitive parameter:** $P_{gc}$ is the most highly regulated variable determining GFR in healthy individuals.

Question 264: A 45-year-old male patient was diagnosed with pituitary stalk compression due to an increase in which hormone secretion?

A. FSH
B. GH
C. TSH
D. Prolactin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Prolactin**. This phenomenon is rooted in the unique neuroendocrine control of the anterior pituitary. **Underlying Concept:** Unlike most anterior pituitary hormones which are primarily regulated by hypothalamic *releasing* factors, **Prolactin is under tonic inhibition by Dopamine** (also known as Prolactin-Inhibiting Hormone). Dopamine is secreted by the hypothalamus and travels through the **hypothalamohyseal portal system** in the pituitary stalk to reach the lactotrophs. When the pituitary stalk is compressed (by a tumor, trauma, or inflammation), the flow of dopamine is interrupted. This "disinhibition" allows prolactin levels to rise—a clinical scenario known as the **"Stalk Effect."** **Why Incorrect Options are Wrong:** * **FSH, GH, and TSH (Options A, B, C):** These hormones require specific hypothalamic *releasing* hormones (GnRH, GHRH, and TRH, respectively) to stimulate their secretion. If the pituitary stalk is compressed, these releasing hormones cannot reach the anterior pituitary, typically leading to a **decrease** (deficiency) in these hormones rather than an increase. **High-Yield Clinical Pearls for NEET-PG:** * **The "Stalk Effect" Rule:** Any lesion that interferes with the hypothalamus or pituitary stalk will cause a **decrease** in all anterior pituitary hormones **EXCEPT Prolactin**, which will **increase**. * **Dopamine Agonists:** Drugs like Bromocriptine and Cabergoline mimic dopamine and are the first-line treatment for hyperprolactinemia. * **Clinical Presentation:** In males, hyperprolactinemia often presents with decreased libido, erectile dysfunction, and occasionally galactorrhea.

Question 265: GFR decreases with the following:

A. Hypoproteinemia
B. Hypotension (Correct Answer)
C. Hypertension
D. All of the above

Explanation: **Explanation:** The Glomerular Filtration Rate (GFR) is determined by the **Starling forces** acting across the glomerular capillaries. The primary driving force for filtration is the **Glomerular Capillary Hydrostatic Pressure ($P_{GC}$)**, which is directly influenced by the systemic arterial blood pressure. **1. Why Hypotension is Correct:** Hypotension (a decrease in systemic blood pressure) leads to a reduction in renal perfusion pressure. This directly lowers the $P_{GC}$. According to the GFR formula: $GFR = K_f \times [(P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS})]$, a drop in $P_{GC}$ reduces the net filtration pressure, thereby decreasing the GFR. While autoregulation (myogenic mechanism and tubuloglomerular feedback) attempts to maintain GFR, severe or persistent hypotension overrides these mechanisms, leading to pre-renal azotemia. **2. Why the other options are incorrect:** * **Hypoproteinemia:** This condition involves a decrease in plasma proteins (e.g., albumin), which lowers the **Glomerular Capillary Oncotic Pressure ($\pi_{GC}$)**. Since oncotic pressure opposes filtration, a decrease in $\pi_{GC}$ actually **increases** the GFR. * **Hypertension:** High systemic blood pressure increases the $P_{GC}$. While renal autoregulation works to keep GFR stable by constricting the afferent arteriole, the physiological tendency of hypertension is to **increase** or maintain GFR, not decrease it. **Clinical Pearls for NEET-PG:** * **Afferent Arteriole:** Constriction decreases GFR; Dilation increases GFR. * **Efferent Arteriole:** Constriction (at moderate levels) increases GFR by backing up pressure; however, severe constriction can decrease GFR due to reduced renal blood flow. * **Starling Forces:** $P_{GC}$ is the only force that significantly favors filtration; $\pi_{GC}$ and $P_{BS}$ (Bowman’s space hydrostatic pressure) are the primary forces opposing it.

Question 266: Which of the following substances is primarily filtered by the glomerulus with minimal secretion or reabsorption in the renal tubules?

A. Sodium
B. Inulin (Correct Answer)
C. Glucose
D. Amino acids

Explanation: ### Explanation **Correct Answer: B. Inulin** Inulin is a fructose polymer that serves as the **gold standard** for measuring the Glomerular Filtration Rate (GFR). Its unique physiological properties make it ideal for this purpose: it is freely filtered at the glomerulus and is neither reabsorbed nor secreted by the renal tubules. Consequently, the amount of inulin filtered is exactly equal to the amount excreted in the urine ($Filtered\ Load = Excretion\ Rate$). **Analysis of Incorrect Options:** * **A. Sodium:** Sodium is freely filtered but undergoes massive reabsorption (approximately 99%) throughout the nephron, primarily in the proximal convoluted tubule (PCT) and the Loop of Henle. * **C. Glucose:** Under normal physiological conditions, glucose is freely filtered but **100% reabsorbed** in the PCT via SGLT-2 and SGLT-1 transporters. It only appears in urine if the blood glucose level exceeds the renal threshold (approx. 180 mg/dL). * **D. Amino acids:** Similar to glucose, amino acids are freely filtered but are almost entirely reabsorbed in the PCT to conserve essential nutrients. **High-Yield Clinical Pearls for NEET-PG:** * **Creatinine vs. Inulin:** While Inulin is the gold standard, **Creatinine** is used clinically to estimate GFR. Note that Creatinine slightly *overestimates* GFR because a small amount is secreted by the tubules. * **Para-aminohippuric acid (PAH):** Unlike inulin, PAH is both filtered and aggressively secreted. It is used to measure **Renal Plasma Flow (RPF)** because it is almost completely cleared from the blood in a single pass. * **Criteria for GFR Marker:** Must be non-toxic, not metabolized by the kidney, and physiologically inert.

Question 267: In response to a change in pH, increased glutaminase activity is due to which of the following?

A. Increased secretion of ammonia (Correct Answer)
B. Increased reabsorption of bicarbonate
C. Increased excretion of H+ ions
D. Increased excretion of OH+ ions

Explanation: **Explanation:** The primary mechanism for the renal compensation of **metabolic acidosis** is the synthesis and secretion of ammonia ($NH_3$). This process occurs predominantly in the **proximal convoluted tubule (PCT)**. **Why Option A is correct:** When the systemic pH drops (acidosis), the enzyme **glutaminase** is upregulated in the mitochondria of PCT cells. Glutaminase deaminates the amino acid **Glutamine** into glutamate and **ammonium ions ($NH_4^+$)**. The $NH_4^+$ then dissociates into $NH_3$ and $H^+$. The ammonia ($NH_3$) is secreted into the tubular lumen, where it acts as a crucial urinary buffer, trapping $H^+$ ions to form $NH_4^+$ (which is then excreted). Therefore, increased glutaminase activity is the direct enzymatic driver for **increased secretion of ammonia**. **Why the other options are incorrect:** * **Option B:** While bicarbonate reabsorption does increase during acidosis, it is a *result* of the overall renal response (specifically via the generation of "new" bicarbonate during glutamine metabolism), not the direct functional outcome of glutaminase activity itself. * **Option C:** $H^+$ excretion increases, but glutaminase specifically facilitates this by providing the $NH_3$ buffer. The question asks what the enzyme activity specifically achieves; its primary biochemical product is ammonia. * **Option D:** $OH^+$ ions do not exist in this physiological context; the body deals with hydroxyl ions ($OH^-$), but they are not the target of renal acid-base regulation. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Action:** Glutaminase activity is highest in the **Proximal Convoluted Tubule**. * **Ammoniagenesis:** For every molecule of Glutamine metabolized, **two $NH_4^+$ ions** are secreted and **two "new" $HCO_3^-$ ions** are returned to the blood. * **Chronic Acidosis:** In chronic states, the kidneys can increase ammonia production 10-fold, making it the most flexible component of the renal buffer system compared to the fixed phosphate buffer.

Question 268: Normal filtration fraction is around:

A. 0.05
B. 0.1
C. 0.2 (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Filtration Fraction (FF)** is the ratio of the Glomerular Filtration Rate (GFR) to the Renal Plasma Flow (RPF). It represents the fraction of blood plasma that is actually filtered across the glomerular capillaries into the Bowman’s space. The formula is: **FF = GFR / RPF** * **Normal GFR:** ~125 mL/min * **Normal RPF:** ~625 mL/min * **Calculation:** 125 / 625 = **0.2 (or 20%)** **Why Option C is correct:** In a healthy adult, approximately 20% of the plasma entering the kidneys is filtered. This value (0.2) is a physiological constant under normal conditions, ensuring adequate clearance of waste while maintaining sufficient peritubular capillary pressure for reabsorption. **Why other options are incorrect:** * **Option A (0.05) and B (0.1):** These values are too low. A filtration fraction this low would indicate either a significant drop in GFR (e.g., acute kidney injury) or an excessive increase in renal plasma flow, leading to insufficient waste excretion. **High-Yield NEET-PG Clinical Pearls:** 1. **Effect of Efferent Arteriole Constriction:** This increases FF. Constriction increases glomerular hydrostatic pressure (increasing GFR) but decreases RPF; since GFR increases more than RPF decreases, the ratio rises. 2. **Hypovolemia/Hemorrhage:** In states of low blood volume, the body maintains GFR via Angiotensin II (which constricts the efferent arteriole). This leads to a **rise in FF** to compensate for decreased renal blood flow. 3. **Peritubular Capillaries:** A high FF increases the oncotic pressure in the peritubular capillaries, which serves as a primary driving force for tubular reabsorption.

Question 269: Free water clearance by the kidney is increased by which of the following?

A. Diabetes insipidus (Correct Answer)
B. Renal failure
C. Diuretic therapy
D. Diabetes mellitus

Explanation: ### Explanation **Concept of Free Water Clearance ($C_{H_2O}$):** Free water clearance represents the volume of solute-free water excreted by the kidneys per unit of time. It is calculated as the difference between total urine flow ($V$) and osmolar clearance ($C_{osm}$): $C_{H_2O} = V - C_{osm}$. * **Positive $C_{H_2O}$:** The kidney is excreting dilute urine (excess water). * **Negative $C_{H_2O}$:** The kidney is concentrating urine (conserving water). **Why Diabetes Insipidus (DI) is Correct:** In DI (whether Central or Nephrogenic), there is either a lack of ADH or a lack of renal response to it. Without ADH, the collecting ducts remain impermeable to water. Large volumes of hypotonic, dilute urine are excreted. Since $V$ is very high and $U_{osm}$ is very low, the **Free Water Clearance increases significantly.** **Why Other Options are Incorrect:** * **Renal Failure:** In chronic kidney disease, the kidney loses its ability to both concentrate and dilute urine (isosthenuria). The $C_{H_2O}$ tends to approach zero. * **Diuretic Therapy:** Most diuretics (like Loop diuretics) interfere with the medullary osmotic gradient. While they increase urine volume, they also increase solute excretion ($C_{osm}$). This typically results in a $C_{H_2O}$ that moves toward zero. * **Diabetes Mellitus:** This causes **osmotic diuresis** due to glucose in the tubules. While urine volume increases, the solute load ($C_{osm}$) is very high, which actually decreases the free water clearance. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Dilution:** The "diluting segments" of the nephron are the Thick Ascending Limb (TAL) and the Distal Convoluted Tubule (DCT). * **ADH Action:** ADH acts on **V2 receptors** in the late distal tubule and collecting ducts to insert **Aquaporin-2** channels. * **Formula:** $C_{H_2O} = V \times (1 - U_{osm}/P_{osm})$. If $U_{osm} < P_{osm}$, $C_{H_2O}$ is positive.

Question 270: Which of the following is not a site of renal tubular secretion?

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

Explanation: **Explanation:** Renal tubular secretion is the process by which substances are moved from the peritubular capillaries into the tubular lumen to be excreted in the urine. While most segments of the nephron participate in this process, the **Collecting Tubule** (specifically the cortical and medullary collecting ducts) is primarily involved in the **reabsorption** of water (via ADH) and urea, and the **fine-tuning** of electrolyte balance, rather than being a primary site for the active secretion of metabolic wastes or foreign substances. * **Proximal Convoluted Tubule (PCT):** This is the most active site for secretion. It secretes organic acids and bases, including bile salts, oxalate, urate, and drugs like penicillin and salicylates. It also secretes $H^+$ ions. * **Distal Convoluted Tubule (DCT):** This segment is crucial for the secretion of $K^+$ and $H^+$ ions, primarily under the influence of aldosterone, to maintain acid-base and electrolyte balance. * **Loop of Henle:** While primarily known for the countercurrent multiplier system, the thin descending limb is involved in the secretion of urea into the tubular fluid (urea recycling). **NEET-PG High-Yield Pearls:** 1. **PCT** is the site for the secretion of **Para-aminohippuric acid (PAH)**, which is used to measure Renal Plasma Flow (RPF). 2. **Potassium ($K^+$)** is unique because it is both reabsorbed (PCT/Loop) and secreted (DCT/Collecting duct). The net urinary excretion of $K^+$ is determined mainly by secretion in the late distal tubule and cortical collecting duct. 3. **Ammonia ($NH_3$)** is synthesized and secreted primarily in the **PCT**.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

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Tubular Reabsorption and Secretion

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Concentration and Dilution of Urine

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Acid-Base Regulation by the Kidneys

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Sodium and Water Balance

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Potassium Regulation

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Calcium and Phosphate Handling

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Micturition Physiology

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Renal Function Tests

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Integrative Responses to Fluid Challenges

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