Renal Physiology Practice Questions

Q: Tamm-Horsfall protein is secreted by epithelial cells of which part of the nephron?

Loop of Henle. **Explanation:** **Tamm-Horsfall protein (THP)**, also known as **Uromodulin**, is the most abundant protein excreted in normal urine. It is a high-molecular-weight glycoprotein synthesized and secreted exclusively by the epithelial cells of the **thick ascending limb (TAL) of the Loop of Henle**. **Why Option B is Correct:** The TAL cells produce THP and anchor it to the luminal membrane. Its primary functions include preventing calcium oxalate stone formation, providing defense against urinary tract infections (UTIs) by binding to *E. coli* fimbriae, and regulating salt transport. Because it is secreted specifically in the TAL, it serves as a specific marker for this segment of the nephron. **Why Other Options are Incorrect:** * **A. Proximal Convoluted Tubule:** This segment is primarily involved in the reabsorption of filtered proteins (like albumin) rather than the secretion of specific structural glycoproteins like THP. * **C. Distal Convoluted Tubule:** While some older texts suggest minimal secretion in the early DCT, the definitive and primary site recognized in high-yield physiology is the TAL of the Loop of Henle. * **D. Glomerulus:** The glomerulus acts as a filtration barrier. THP is not filtered from the blood; it is added to the tubular fluid after the filtrate has passed the glomerulus. **Clinical Pearls for NEET-PG:** 1. **Cast Formation:** THP forms the **matrix of all urinary casts**. In conditions of low flow or high acidity, it gels to trap cells (e.g., RBC casts, WBC casts). 2. **Hyaline Casts:** These are composed almost entirely of Tamm-Horsfall protein. 3. **Myeloma Kidney:** Bence-Jones proteins can precipitate with THP, leading to intratubular obstruction and renal failure. 4. **Genetic Link:** Mutations in the Uromodulin gene are associated with Medullary Cystic Kidney Disease.

Q: Which of the following substances is reabsorbed along with sodium in the early portion of the proximal tubule?

All of the above. **Explanation:** In the **early proximal convoluted tubule (PCT)**, the reabsorption of sodium is coupled with several essential solutes through specific secondary active transport mechanisms. This process is driven by the sodium-potassium ATPase pump on the basolateral membrane, which creates a low intracellular sodium concentration and a negative membrane potential. 1. **Glucose:** It is reabsorbed via **SGLT-2** (Sodium-Glucose Co-transporter 2) in the early PCT. This is a symport mechanism where glucose moves against its gradient along with sodium. 2. **Amino Acids:** Various sodium-dependent amino acid transporters (symporters) facilitate the reabsorption of almost 100% of filtered amino acids in this segment. 3. **Bicarbonate ($HCO_3^-$):** Sodium reabsorption is coupled with hydrogen ion secretion via the **NHE3** (Sodium-Hydrogen Exchanger). The secreted $H^+$ reacts with filtered $HCO_3^-$ to form $CO_2$ and $H_2O$ (catalyzed by Carbonic Anhydrase), which then enters the cell. Thus, $HCO_3^-$ reabsorption is indirectly but fundamentally linked to sodium transport in the early PCT. **Why "All of the above" is correct:** The early PCT is the primary site for "bulk reabsorption." While the late PCT focuses more on sodium chloride (NaCl) reabsorption, the early portion is specialized for the co-transport of sodium with organic solutes (glucose, amino acids) and bicarbonate. **High-Yield Clinical Pearls for NEET-PG:** * **Fanconi Syndrome:** A generalized dysfunction of the PCT leading to the loss of glucose, amino acids, bicarbonate, and phosphates in the urine. * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** These drugs act specifically in the early PCT to treat Diabetes Mellitus by promoting glucosuria. * **Carbonic Anhydrase Inhibitors (Acetazolamide):** Act in the PCT to inhibit $HCO_3^-$ reabsorption, leading to alkaline urine and metabolic acidosis.

Q: Constriction of efferent arteriole produces which of the following effects?

All of the above. **Explanation:** The constriction of the efferent arteriole is a key physiological mechanism for regulating the Glomerular Filtration Rate (GFR) and tubular reabsorption. 1. **Biphasic response on GFR:** At low to moderate levels of constriction, GFR increases because the outflow resistance raises the glomerular hydrostatic pressure ($P_{GC}$). However, at high levels of constriction, the GFR actually decreases. This is because severe constriction significantly reduces Renal Plasma Flow (RPF), causing the filtration fraction to rise so sharply that the plasma oncotic pressure ($π_{GC}$) increases rapidly along the capillary, eventually opposing filtration. 2. **Increased peritubular oncotic pressure:** As blood passes through the glomerulus, fluid is filtered out, leaving behind concentrated plasma proteins. Efferent constriction increases the filtration fraction, meaning a higher proportion of plasma is filtered. Consequently, the blood entering the peritubular capillaries has a significantly higher colloid osmotic (oncotic) pressure. 3. **Increased peritubular absorption:** Tubular reabsorption is governed by Starling forces in the peritubular capillaries. The increased oncotic pressure (noted above) combined with a *decreased* peritubular hydrostatic pressure (due to the resistance at the efferent arteriole) creates a powerful pressure gradient that favors the movement of fluid from the interstitium back into the capillaries. **High-Yield Facts for NEET-PG:** * **Angiotensin II** preferentially constricts the efferent arteriole to maintain GFR during states of low renal perfusion (e.g., dehydration). * **ACE Inhibitors** block this mechanism, leading to efferent vasodilation, which can cause a precipitous drop in GFR in patients with renal artery stenosis. * **Filtration Fraction (FF) = GFR / RPF.** Efferent constriction always increases FF because it decreases RPF more than it affects GFR.

Q: ANP acts at which part of the nephron?

Collecting tubule. **Explanation:** Atrial Natriuretic Peptide (ANP) is a potent hormone secreted by the cardiac atria in response to increased stretch (volume overload). Its primary goal is to promote **natriuresis** (sodium excretion) and **diuresis** (water excretion) to reduce blood volume. **Why the Collecting Tubule is correct:** The principal site of action for ANP is the **Inner Medullary Collecting Duct (IMCD)**. ANP binds to its receptors (NPR-A), which increases intracellular **cGMP**. This second messenger inhibits the apical sodium channels (**ENaC**) and the basolateral Na+/K+ ATPase. By blocking sodium reabsorption in this final segment of the nephron, ANP ensures that sodium and water are excreted in the urine. **Why the other options are incorrect:** * **Proximal Tubule:** While some studies suggest minor inhibitory effects on Na-H exchange, the PT is not the primary physiological target for ANP’s natriuretic effect. * **Distal Tubule:** This segment is primarily regulated by Aldosterone and Thiazide-sensitive transporters; it is not the major site for ANP action. * **Henle Loop:** The Thick Ascending Limb is primarily involved in the countercurrent multiplier system and is regulated by different mechanisms (e.g., NKCC2), not ANP. **High-Yield Clinical Pearls for NEET-PG:** 1. **Second Messenger:** ANP and NO (Nitric Oxide) both act via **cGMP**. 2. **Afferent/Efferent Effect:** In the glomerulus, ANP **dilates the afferent arteriole** and **constricts the efferent arteriole**, increasing the GFR (Glomerular Filtration Rate). 3. **Antagonist Role:** ANP acts as a physiological antagonist to the **Renin-Angiotensin-Aldosterone System (RAAS)**. 4. **BNP:** Brain Natriuretic Peptide (secreted by ventricles) acts similarly and is a key clinical marker for diagnosing Heart Failure.

Q: The glomerular filtration rate of the human kidney may be determined by measuring the plasma clearance of which substance?

Inulin. ### Explanation **Why Inulin is the Correct Answer:** To measure the **Glomerular Filtration Rate (GFR)**, a substance must be **freely filtered** at the glomerulus and must be **neither reabsorbed nor secreted** by the renal tubules. **Inulin**, a fructose polymer, perfectly meets these criteria. Since every molecule of inulin filtered into the Bowman’s capsule ends up in the urine, its clearance rate ($C = UV/P$) is exactly equal to the GFR. It is considered the **gold standard** for measuring GFR. **Analysis of Incorrect Options:** * **PAH (Para-aminohippuric acid):** PAH is both filtered and extensively secreted by the tubules. Because it is almost completely cleared from the blood in a single pass through the kidney, its clearance is used to measure **Effective Renal Plasma Flow (ERPF)**, not GFR. * **Urea:** While urea is freely filtered, it undergoes significant **passive reabsorption** (about 50%) in the tubules. Therefore, urea clearance underestimates the true GFR. * **Glucose:** In a healthy individual, glucose is freely filtered but **completely reabsorbed** in the proximal convoluted tubule. Its clearance is normally **zero**, making it useless for measuring GFR. **High-Yield 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. * **Filtration Fraction (FF):** Calculated as $GFR / RPF$. Normal FF is approximately **20%**. * **Criteria for GFR Marker:** Must be non-toxic, not metabolized by the kidney, and not protein-bound.

Q: A 24-year-old woman with a large appetite for salt consumes approximately 25g of salt each day. What is the approximate amount of salt (in grams) that is excreted each day by her kidneys?

23. **Explanation** The core concept tested here is **Sodium Balance**. In a healthy individual with normal renal function, the body maintains a steady state where the total output of sodium equals the total intake. 1. **Why Option C is correct:** The patient consumes **25g** of salt daily. Under normal physiological conditions, sodium is excreted through three main routes: the kidneys (urine), the skin (sweat), and the gastrointestinal tract (feces). In a temperate environment with normal activity, approximately **90-95%** of ingested salt is excreted via the kidneys, while the remaining 5-10% is lost through sweat and feces. * Calculation: 90-95% of 25g ≈ **22.5 to 23.75g**. * Therefore, **23g** is the most accurate approximation of renal excretion to maintain homeostasis. 2. **Why other options are incorrect:** * **Option A (4g) & B (12g):** These values are significantly lower than the intake. If a person consumes 25g but only excretes 4g or 12g, they would develop massive positive sodium balance, leading to severe fluid retention, edema, and hypertension. * **Option D (50g):** This represents a negative sodium balance. Excreting more than the intake would lead to volume depletion and hyponatremia, which does not occur in a healthy individual at steady state. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Handling:** 65% of filtered sodium is reabsorbed in the Proximal Convoluted Tubule (PCT). * **Aldosterone:** The primary hormone regulating the "final touch" of sodium excretion in the distal nephron. * **Pressure Natriuresis:** An increase in arterial pressure directly increases sodium excretion to maintain balance. * **Steady State Rule:** Intake = Output. If intake increases, renal excretion must increase proportionally within 24–48 hours to prevent volume overload.

Q: In which segment of the nephron does tubular fluid have the highest osmolarity?

Henle's loop. **Explanation:** The osmolarity of tubular fluid changes significantly as it traverses the nephron due to the **countercurrent multiplier system**. **1. Why Henle’s Loop is Correct:** The highest osmolarity is reached at the **tip of the Loop of Henle** (the hairpin bend). As fluid descends the thin descending limb, it loses water to the hypertonic medullary interstitium but remains impermeable to solutes. This concentrates the tubular fluid, reaching a peak osmolarity of approximately **1200–1400 mOsm/L** in long-looped juxtamedullary nephrons. **2. Why the Other Options are Incorrect:** * **Proximal Tubule:** Fluid here is **isosmotic** to plasma (~300 mOsm/L) because water and solutes are reabsorbed in equal proportions. * **Distal Tubule:** This segment is part of the "diluting segment." Since solutes (NaCl) are reabsorbed without water, the fluid becomes **hypoosmotic** (~100 mOsm/L). * **Collecting Duct:** While the collecting duct passes through the hypertonic medulla, its primary role is regulated water reabsorption via ADH. Even under maximal ADH influence, the fluid osmolarity equilibrates with the medullary tip but does not exceed it. **High-Yield NEET-PG Pearls:** * **Descending Limb:** Permeable to water, impermeable to NaCl (Concentrating segment). * **Ascending Limb:** Impermeable to water, active reabsorption of NaCl (Diluting segment). * **Vasa Recta:** Acts as a **countercurrent exchanger** to maintain the medullary osmotic gradient without washing it out. * **Urea Recycling:** Contributes nearly 40-50% of the hyperosmolarity in the inner medullary interstitium.

Q: What is the most important buffer system present in the distal convoluted tubule?

Ammonia. **Explanation:** The primary role of the distal nephron in acid-base balance is the excretion of fixed acids ($H^+$ ions). While the bicarbonate buffer system is crucial in the extracellular fluid and proximal tubule, it is not the dominant buffer in the distal tubule because most bicarbonate has already been reabsorbed. **Why Ammonia is the Correct Answer:** The **Ammonia ($NH_3/NH_4^+$) buffer system** is the most important and physiologically flexible buffer in the distal tubule. Unlike other buffers, its synthesis can be **upregulated** in response to chronic acidosis. Ammonia is produced in the proximal tubule from Glutamine, but it exerts its major buffering effect in the collecting ducts. $NH_3$ diffuses into the tubular lumen, combines with secreted $H^+$ to form $NH_4^+$ (ammonium), which is "trapped" and excreted. This system accounts for about **60-75%** of net acid excretion. **Analysis of Incorrect Options:** * **Bicarbonate:** While it is the most important ECF buffer, 80-90% is reabsorbed in the proximal tubule. By the time fluid reaches the distal tubule, its concentration is too low to be the primary buffer. * **Phosphate:** Known as the "Titratable Acid" buffer. Although important, its concentration is fixed and cannot be increased during acidosis, making it less significant than the ammonia system. * **Protein:** Proteins are major intracellular and plasma buffers but are normally absent (or present in negligible amounts) in the tubular fluid of a healthy nephron. **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The production of ammonia from **Glutamine** by the enzyme **Glutaminase** is the key adaptive response to metabolic acidosis. * **Diffusion Trapping:** $NH_3$ is lipid-soluble and diffuses easily; $NH_4^+$ is water-soluble and trapped in the lumen (cannot diffuse back). * **Net Acid Excretion (NAE):** Calculated as $[NH_4^+ \text{ excretion} + \text{Titratable Acid}] - [\text{Bicarbonate excretion}]$.

Q: What percentage of glucose is absorbed in the proximal convoluted tubule?

100%. **Explanation:** In a healthy individual, **100% of filtered glucose is reabsorbed in the Proximal Convoluted Tubule (PCT)**. Glucose is a vital nutrient, and the kidneys are designed to ensure none is lost in the urine under normal physiological conditions. **Mechanism:** Reabsorption occurs via **Secondary Active Transport**. * **SGLT-2 (Sodium-Glucose Co-transporter 2):** Located in the early PCT (S1 segment), it reabsorbs about 90% of glucose. * **SGLT-1:** Located in the late PCT (S3 segment), it reabsorbs the remaining 10%. * Once inside the tubular cell, glucose exits into the blood via facilitated diffusion through **GLUT-2** (early PCT) and **GLUT-1** (late PCT). **Analysis of Incorrect Options:** * **B (0%):** This would imply total glycosuria, seen only in severe pathological states or specific drug actions. * **C & D (60% & 50%):** While the PCT reabsorbs roughly 65% of water and electrolytes (like Na+ and K+), it is specialized to reabsorb 100% of glucose and amino acids. **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Threshold for Glucose:** Glucose starts appearing in urine (glycosuria) when blood glucose levels exceed **180 mg/dL**. 2. **Transport Maximum ($T_m$):** The point where all glucose transporters are saturated. For men, $T_mG$ is approx. **375 mg/min**; for women, **300 mg/min**. 3. **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A modern class of anti-diabetic drugs that work by inhibiting glucose reabsorption in the PCT, purposely inducing glycosuria to lower blood sugar. 4. **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, and phosphates in the urine.

Q: A luminal cell membrane Na+ channel is the main pathway for Na+ reabsorption in which of the following cell types?

Collecting duct principal cells. **Explanation:** The correct answer is **Collecting duct principal cells**. The question asks for the cell type where Na+ reabsorption occurs via a **luminal Na+ channel** (rather than a symporter or antiporter). 1. **Why Principal Cells are correct:** In the late distal tubule and cortical collecting duct, Na+ reabsorption is mediated by the **Epithelial Sodium Channel (ENaC)** located on the apical (luminal) membrane. This is a selective channel, not a transporter. This process is electrogenic and is the primary site of action for **Aldosterone**, which increases the number and activity of ENaCs. 2. **Why other options are incorrect:** * **Proximal Tubule Cells:** Na+ reabsorption here occurs primarily via the **Na+-H+ exchanger (NHE3)** and various symporters (e.g., Na+-Glucose, Na+-Amino Acid). * **Thick Ascending Limb (TAL) Cells:** Na+ enters these cells via the **NKCC2 symporter** (Na+-K+-2Cl- cotransporter), which is the target of loop diuretics. * **Distal Convoluted Tubule (DCT) Cells:** Na+ reabsorption is mediated by the **NCC symporter** (Na+-Cl- cotransporter), which is inhibited by thiazide diuretics. **High-Yield Clinical Pearls for NEET-PG:** * **Liddle Syndrome:** A genetic mutation causing "gain of function" in ENaC, leading to hypertension, hypokalemia, and metabolic alkalosis (mimics hyperaldosteronism but with low renin/aldosterone). * **Amiloride & Triamterene:** These are potassium-sparing diuretics that specifically block the **ENaC** in the principal cells. * **ANP (Atrial Natriuretic Peptide):** Inhibits ENaC, promoting natriuresis.

Question 1

Tamm-Horsfall protein is secreted by epithelial cells of which part of the nephron?

Accepted Answer

Loop of Henle

Answer

Proximal convoluted tubule

Answer

Distal convoluted tubule

Answer

Glomerulus

Question 2

Which of the following substances is reabsorbed along with sodium in the early portion of the proximal tubule?

Accepted Answer

All of the above

Answer

Glucose

Answer

Amino acids

Answer

Bicarbonate

Question 3

Constriction of efferent arteriole produces which of the following effects?

Accepted Answer

All of the above

Answer

Biphasic response on GFR

Answer

Increased peritubular oncotic pressure

Answer

Increased peritubular absorption

Question 4

ANP acts at which part of the nephron?

Accepted Answer

Collecting tubule

Answer

Proximal tubule

Answer

Distal tubule

Answer

Henle loop

Question 5

The glomerular filtration rate of the human kidney may be determined by measuring the plasma clearance of which substance?

Accepted Answer

Inulin

Answer

PAH

Answer

Urea

Answer

Glucose

Question 6

A 24-year-old woman with a large appetite for salt consumes approximately 25g of salt each day. What is the approximate amount of salt (in grams) that is excreted each day by her kidneys?

Accepted Answer

23

Answer

4

Answer

12

Answer

50

Question 7

In which segment of the nephron does tubular fluid have the highest osmolarity?

Accepted Answer

Henle's loop

Answer

Distal tubule

Answer

Proximal tubule

Answer

Collecting duct

Question 8

What is the most important buffer system present in the distal convoluted tubule?

Accepted Answer

Ammonia

Answer

Bicarbonate

Answer

Phosphate

Answer

Protein

Question 9

What percentage of glucose is absorbed in the proximal convoluted tubule?

Accepted Answer

100%

Answer

0%

Answer

60%

Answer

50%

Question 10

A luminal cell membrane Na+ channel is the main pathway for Na+ reabsorption in which of the following cell types?

Accepted Answer

Collecting duct principal cells

Answer

Proximal tubule cells

Answer

Thick ascending limb cells

Answer

Distal convoluted tubule cells

Renal Physiology — MCQs

Renal Physiology — MCQs

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