Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 271: Tamm-Horsfall protein is secreted by epithelial cells of which part of the nephron?

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

Explanation: **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.

Question 272: Which of the following substances is reabsorbed along with sodium in the early portion of the proximal tubule?

A. Glucose
B. Amino acids
C. Bicarbonate
D. All of the above (Correct Answer)

Explanation: **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.

Question 273: Which of the following is NOT a component of the juxtaglomerular apparatus?

A. Lacis cells
B. Macula densa
C. Juxtaglomerular cells
D. Glomerular mesangial cells (Correct Answer)

Explanation: ### Explanation The **Juxtaglomerular Apparatus (JGA)** is a specialized structure located at the vascular pole of the glomerulus, where the distal convoluted tubule (DCT) comes into contact with the afferent and efferent arterioles. Its primary function is the regulation of systemic blood pressure and glomerular filtration rate (GFR). #### Why Glomerular Mesangial Cells are the Correct Answer: **Glomerular mesangial cells** (also known as intrinsic or intraglomerular mesangial cells) are located *inside* the glomerular capillary loops. While they provide structural support and have contractile properties, they are **not** considered a structural component of the JGA. #### Analysis of Other Options: * **Macula Densa:** These are specialized epithelial cells in the thick ascending limb/DCT. They act as **chemoreceptors** that sense changes in sodium chloride (NaCl) concentration in the tubular fluid. * **Juxtaglomerular (JG) Cells:** These are modified smooth muscle cells located primarily in the wall of the **afferent arteriole**. They act as **baroreceptors** and are responsible for the synthesis, storage, and release of **renin**. * **Lacis Cells:** Also known as **Extraglomerular Mesangial Cells** (or Polkissen cells), they are located in the triangular space between the arterioles and the macula densa. They facilitate communication between the macula densa and JG cells. #### NEET-PG High-Yield Pearls: * **Tubuloglomerular Feedback (TGF):** This is the mechanism where the Macula Densa senses high NaCl (indicating high GFR) and causes vasoconstriction of the afferent arteriole to normalize GFR. * **Renin Release:** Stimulated by decreased renal perfusion pressure (baroreceptors), decreased NaCl delivery (chemoreceptors), and sympathetic stimulation ($\beta_1$ receptors). * **Histology Tip:** JG cells contain prorenin and renin granules and are considered the "endocrine" component of the kidney.

Question 274: What is the primary reason for the buffering action of hemoglobin?

A. Heme
B. Porphyrin
C. Imidazole group of histidine (Correct Answer)
D. Histidine group of imidazole

Explanation: **Explanation:** The buffering capacity of hemoglobin (Hb) is primarily attributed to the **imidazole group of the amino acid histidine**. **1. Why the correct answer is right:** Hemoglobin contains 38 histidine residues per molecule. The imidazole group of histidine has a **pKa of approximately 6.8**, which is very close to the physiological pH of blood (7.4). According to the Henderson-Hasselbalch principle, a buffer is most effective when its pKa is near the pH of the medium. This allows the imidazole group to reversibly bind or release hydrogen ions ($H^+$), effectively neutralizing metabolic acids. Furthermore, deoxygenated hemoglobin (Deoxy-Hb) is a better buffer than oxyhemoglobin because its pKa increases, making it more basic and allowing it to carry more $H^+$ ions (the **Haldane Effect**). **2. Why the incorrect options are wrong:** * **Heme & Porphyrin:** These are the prosthetic groups responsible for oxygen binding. While they are structural components of hemoglobin, they do not possess the ionizable side chains necessary for significant pH buffering. * **Histidine group of imidazole:** This is a play on words. Imidazole is a chemical functional group *within* the amino acid histidine, not the other way around. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Most important intracellular buffer:** Hemoglobin (due to its high concentration and histidine content). * **Most important extracellular/plasma buffer:** Bicarbonate ($HCO_3^-$) system. * **Isohydric Principle:** All buffer systems in a compartment (like blood) are in equilibrium with the same $[H^+]$; therefore, a change in one system affects all others. * **Bohr Effect:** Increased $H^+$ (acidity) decreases Hb's affinity for $O_2$, shifting the dissociation curve to the right.

Question 275: ANP acts at which part of the nephron?

A. Proximal tubule
B. Distal tubule
C. Collecting tubule (Correct Answer)
D. Henle loop

Explanation: **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.

Question 276: What is the normal pH range of urine?

A. 7.10 - 7.40
B. 3 - 3.5
C. 4 - 6.5 (Correct Answer)
D. 6.5 - 7.10

Explanation: **Explanation:** The pH of urine is a reflection of the kidney's vital role in maintaining acid-base homeostasis. The kidneys regulate the plasma pH (7.35–7.45) by excreting hydrogen ions ($H^+$) and reabsorbing bicarbonate ($HCO_3^-$). **Why Option C is Correct:** The normal range for urine pH is typically **4.5 to 8.0**, with an average of **6.0** (slightly acidic). In the context of the provided options, **4 – 6.5** is the most accurate representation of the physiological range. The lower limit of urine pH is approximately **4.5**; the kidneys cannot pump $H^+$ against a gradient once the tubular fluid reaches this "limiting pH." **Analysis of Incorrect Options:** * **Option A (7.10 - 7.40):** This range is too narrow and alkaline. It mimics the physiological pH of blood, not urine. * **Option B (3 - 3.5):** This is extremely acidic. The human kidney is incapable of acidifying urine below a pH of 4.4–4.5 due to the limitations of the $H^+$-ATPase pumps in the intercalated cells. * **Option D (6.5 - 7.10):** While urine can be in this range (especially after a meal, known as the "alkaline tide"), it does not account for the normal acidic variations produced by a standard protein-rich diet. **High-Yield Clinical Pearls for NEET-PG:** 1. **Limiting pH:** The minimum urine pH achievable by the human kidney is **4.5**. 2. **Dietary Impact:** High-protein diets (acid ash) decrease urine pH, while vegetarian diets (alkaline ash) increase it. 3. **Renal Tubular Acidosis (RTA):** In Type 1 (Distal) RTA, the kidney fails to secrete $H^+$, meaning the urine pH **cannot** fall below 5.5 despite systemic acidosis. 4. **Stones:** Acidic urine promotes **Uric acid** and **Cystine** stones; alkaline urine promotes **Calcium phosphate** and **Struvite** (triple phosphate) stones.

Question 277: Which part of the nephron is impermeable to water?

A. Ascending limb of loop of Henle (Correct Answer)
B. Collecting duct
C. Proximal convoluted tubule
D. Descending limb of loop of Henle

Explanation: ***Ascending limb of loop of Henle*** - The ascending limb is **impermeable to water** due to the absence of **aquaporin water channels**, making it the only segment that cannot reabsorb water. - This impermeability allows active transport of **sodium** and **chloride** without water following, creating the **countercurrent multiplier** mechanism essential for urine concentration. *Collecting duct* - The collecting duct has **variable water permeability** regulated by **ADH (antidiuretic hormone)** through aquaporin-2 channels. - When ADH is present, it becomes **highly permeable to water**, allowing final urine concentration adjustments. *Proximal convoluted tubule* - The PCT is **highly permeable to water** through constitutively active **aquaporin-1 channels**, reabsorbing approximately **65% of filtered water**. - Water reabsorption here occurs **passively** following active sodium transport, maintaining **isotonic** fluid. *Descending limb of loop of Henle* - The descending limb is **freely permeable to water** via **aquaporin-1 channels** but relatively impermeable to solutes. - This allows water to be **reabsorbed** as the filtrate moves toward the hypertonic medullary interstitium, concentrating the tubular fluid.

Question 278: The glomerular filtration rate of the human kidney may be determined by measuring the plasma clearance of which substance?

A. PAH
B. Urea
C. Inulin (Correct Answer)
D. Glucose

Explanation: ### 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.

Question 279: 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?

A. 4
B. 12
C. 23 (Correct Answer)
D. 50

Explanation: **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.

Question 280: In which segment of the nephron does tubular fluid have the highest osmolarity?

A. Henle's loop (Correct Answer)
B. Distal tubule
C. Proximal tubule
D. Collecting duct

Explanation: **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.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

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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