Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 91: Which of the following is not a renal vasoconstrictor?

A. Norepinephrine
B. Acetylcholine (Correct Answer)
C. Angiotensin II
D. None of the above

Explanation: **Explanation:** The regulation of renal blood flow (RBF) and glomerular filtration rate (GFR) is governed by various hormones and autacoids that act as either vasoconstrictors or vasodilators. **Why Acetylcholine is the Correct Answer:** Acetylcholine is a potent **vasodilator**. When it binds to muscarinic receptors on the vascular endothelium, it triggers the release of **Nitric Oxide (NO)**. Nitric oxide diffuses into the vascular smooth muscle cells, causing relaxation and subsequent vasodilation. Therefore, acetylcholine decreases renal vascular resistance and increases renal blood flow, making it the correct choice as it is not a vasoconstrictor. **Analysis of Incorrect Options:** * **A. Norepinephrine:** Released by the sympathetic nervous system, it acts on **α1-adrenoceptors** on renal blood vessels to cause potent vasoconstriction. This reduces both RBF and GFR, especially during physiological stress or hemorrhage. * **C. Angiotensin II:** This is one of the body's most powerful vasoconstrictors. While it preferentially constricts the **efferent arteriole** (to maintain GFR when renal perfusion is low), in higher concentrations, it constricts both afferent and efferent arterioles, increasing renal vascular resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Potent Renal Vasoconstrictors:** Norepinephrine, Epinephrine, Angiotensin II, Endothelin, and ADH (Vasopressin). * **Potent Renal Vasodilators:** Acetylcholine, Nitric Oxide, Prostaglandins (PGE2, PGI2), Bradykinin, and Atrial Natriuretic Peptide (ANP). * **Clinical Correlation:** NSAIDs inhibit prostaglandin synthesis, leading to unopposed vasoconstriction of the afferent arteriole, which can precipitate acute renal failure in susceptible patients. * **Dopamine Paradox:** At low doses, dopamine causes renal vasodilation (via D1 receptors), but at high doses, it causes vasoconstriction (via α1 receptors).

Question 92: Lacis cells are located at?

A. Juxtaglomerular apparatus (JGA) (Correct Answer)
B. Proximal tubule
C. Distal tubule
D. Loop of Henle

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. It consists of three primary components: 1. **Macula Densa:** Specialized cells in the DCT that act as chemoreceptors for sodium chloride. 2. **Juxtaglomerular (JG) Cells:** Modified smooth muscle cells in the afferent arteriole that secrete renin. 3. **Lacis Cells (Extraglomerular Mesangial Cells):** These are located in the triangular space between the afferent arteriole, efferent arteriole, and the macula densa. They are continuous with the intraglomerular mesangial cells and are thought to facilitate signaling between the macula densa and JG cells. **Why incorrect options are wrong:** * **Proximal Tubule:** This is the site of bulk reabsorption (glucose, amino acids, 65% of water/electrolytes) but does not contain JGA components. * **Distal Tubule:** While the *macula densa* is part of the early DCT, Lacis cells themselves are located outside the tubule in the interstitial space of the JGA. * **Loop of Henle:** This segment is primarily involved in the countercurrent multiplier system and concentration of urine; it does not house the JGA. **High-Yield Facts for NEET-PG:** * **Alternative Names:** Lacis cells are also known as **Polkissen cells** or **Goormaghtigh cells**. * **Function:** They play a role in **tubuloglomerular feedback**, helping regulate GFR and systemic blood pressure. * **Histology:** They are characterized by their pale-staining cytoplasm and are structurally similar to smooth muscle cells (containing actin and myosin).

Question 93: Severe anxiety causes which of the following acid-base disturbances?

A. Respiratory acidosis
B. Respiratory alkalosis (Correct Answer)
C. Metabolic acidosis
D. Metabolic alkalosis

Explanation: **Explanation:** **Why Respiratory Alkalosis is the correct answer:** Severe anxiety or panic attacks trigger the sympathetic nervous system, leading to **hyperventilation** (increased rate and depth of breathing). This excessive ventilation results in the "washing out" of Carbon Dioxide ($CO_2$) from the lungs. Since $CO_2$ acts as a volatile acid in the blood (forming carbonic acid), its rapid depletion leads to a decrease in the partial pressure of arterial $CO_2$ ($PaCO_2 < 35\ mmHg$) and a subsequent **increase in blood pH**, resulting in respiratory alkalosis. **Analysis of Incorrect Options:** * **Respiratory Acidosis:** This occurs due to **hypoventilation** (e.g., COPD, opioid overdose), where $CO_2$ is retained, lowering the pH. Anxiety causes the exact opposite (hyperventilation). * **Metabolic Acidosis:** This is caused by a primary decrease in bicarbonate ($HCO_3^-$) or an accumulation of fixed acids (e.g., Diabetic Ketoacidosis, Renal failure). Anxiety does not acutely affect metabolic acid production. * **Metabolic Alkalosis:** This results from a primary gain of $HCO_3^-$ or loss of $H^+$ ions (e.g., persistent vomiting, diuretic use), which is unrelated to the respiratory changes seen in anxiety. **Clinical Pearls for NEET-PG:** * **The "Paper Bag" Treatment:** Rebreathing into a paper bag during an anxiety attack helps the patient inhale expired $CO_2$, restoring $PaCO_2$ levels and correcting the pH. * **Chvostek’s and Trousseau’s Signs:** Acute respiratory alkalosis causes a shift of ionized calcium to bind with albumin. This sudden **hypocalcemia** leads to carpopedal spasms and tetany, often seen in hyperventilating patients. * **Rule of Thumb:** In acute respiratory alkalosis, for every 10 mmHg decrease in $PaCO_2$, the $pH$ increases by approximately 0.08 units.

Question 94: The highest percentage of glomerular filtrate reabsorption occurs in which part of the nephron?

A. Bowman's capsule
B. Proximal tubule (Correct Answer)
C. Thick ascending limb of loop of Henle
D. Distal tubule

Explanation: **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for the reabsorption of glomerular filtrate. Approximately **65-70%** of the total filtered water and sodium, and nearly 100% of glucose and amino acids, are reabsorbed here. This high capacity is due to the presence of a dense "brush border" of microvilli, which significantly increases the surface area for transport, and a high density of mitochondria to provide energy for active transport. **Analysis of Options:** * **A. Bowman’s Capsule:** This is the site of **ultrafiltration**, not reabsorption. It collects the filtrate from the glomerulus and directs it into the PCT. * **C. Thick Ascending Limb (TAL):** This segment reabsorbs about **15-25%** of filtered sodium. It is known as the "diluting segment" because it is impermeable to water but actively transports solutes. * **D. Distal Tubule:** The distal convoluted tubule and collecting ducts are responsible for "fine-tuning" the urine composition, reabsorbing only about **5-10%** of the filtrate, largely under hormonal control (Aldosterone and ADH). **High-Yield NEET-PG Pearls:** 1. **Isotonic Reabsorption:** Reabsorption in the PCT is **iso-osmotic**; the fluid remaining at the end of the PCT has the same osmolarity as plasma (300 mOsm/L). 2. **Obligatory Water Reabsorption:** Water reabsorption in the PCT follows solutes passively and is independent of ADH. 3. **Carbonic Anhydrase:** This enzyme is highly active in the PCT, making it the site of action for **Acetazolamide**. 4. **SGLT-2:** Located in the PCT, these transporters are responsible for glucose reabsorption and are the target of "Gliflozin" drugs used in diabetes.

Question 95: What is the important role of the distal tubule of the kidney in acid-base balance?

A. Secretion of Ammonia (Correct Answer)
B. Secretion of bicarbonate
C. Secretion of HCl
D. Absorption of Ammonia

Explanation: The kidney plays a vital role in maintaining acid-base homeostasis through the excretion of hydrogen ions ($H^+$). However, the minimum urinary pH is approximately 4.5; to excrete more acid, $H^+$ must be buffered. ### **Why "Secretion of Ammonia" is Correct** In the distal tubule (specifically the late distal tubule and collecting ducts), ammonia ($NH_3$) is the most important adaptive buffer. $NH_3$ is lipid-soluble and diffuses from the tubular cells into the lumen. There, it combines with secreted $H^+$ ions to form ammonium ($NH_4^+$). Because $NH_4^+$ is water-soluble and membrane-impermeable (**"ion trapping"**), it is excreted in the urine. This process allows for the excretion of large amounts of metabolic acid without further lowering the urinary pH. ### **Analysis of Incorrect Options** * **B. Secretion of bicarbonate:** The kidney primarily **reabsorbs** filtered bicarbonate (mostly in the proximal tubule) and generates "new" bicarbonate in the distal tubule to replenish the body's alkaline reserve. Secretion only occurs in specific states (via Type B intercalated cells) but is not the primary role in acid-base balance. * **C. Secretion of HCl:** The kidney does not secrete hydrochloric acid directly; it secretes $H^+$ ions which then pair with anions like $Cl^-$ or buffers. * **D. Absorption of Ammonia:** Ammonia is produced and **secreted** (not absorbed) to facilitate acid excretion. ### **NEET-PG High-Yield Pearls** * **Site of Production:** Most ammonia is produced in the **Proximal Convoluted Tubule (PCT)** from the amino acid **Glutamine**. * **Titratable Acidity:** Refers to $H^+$ buffered by phosphates ($HPO_4^{2-}$). Unlike ammonia, phosphate levels are relatively fixed and cannot increase significantly during chronic acidosis. * **Clinical Correlation:** In **Distal Renal Tubular Acidosis (Type 1 RTA)**, the distal tubule fails to secrete $H^+$, leading to an inability to acidify urine (pH remains > 5.5).

Question 96: Which substance acts as a vasodilator in most parts of the body but causes vasoconstriction specifically in the afferent arteriole of the kidney?

A. Nitric oxide
B. Adenosine (Correct Answer)
C. Bradykinin
D. Endothelin

Explanation: **Explanation:** The correct answer is **Adenosine**. This substance exhibits a unique "paradoxical" effect in the kidney compared to the rest of the systemic circulation. **1. Why Adenosine is Correct:** In most systemic tissues (like the heart or skeletal muscle), adenosine acts as a potent vasodilator to increase blood flow during metabolic demand. However, in the kidney, adenosine is the primary mediator of **Tubuloglomerular Feedback (TGF)**. When there is an increase in NaCl delivery to the *macula densa*, adenosine is released. It binds to **A1 receptors** on the afferent arteriole, causing **vasoconstriction**. This reduces the Glomerular Filtration Rate (GFR) to prevent excessive fluid loss. **2. Why the Other Options are Incorrect:** * **Nitric Oxide (NO):** A potent vasodilator in both systemic and renal circulations. It counteracts the effects of Angiotensin II in the kidney. * **Bradykinin:** A vasodilator that acts by stimulating NO release and prostaglandins. It dilates both afferent and efferent arterioles. * **Endothelin:** A powerful vasoconstrictor in almost all vascular beds, including the kidney. It does not show the site-specific reversal seen with adenosine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Receptor Specificity:** Adenosine causes vasoconstriction via **A1 receptors** (afferent arteriole) but can cause vasodilation in the efferent arteriole via **A2 receptors**. * **TGF Mechanism:** Increased GFR → Increased NaCl at Macula Densa → Adenosine release → Afferent vasoconstriction → Decreased GFR. * **Theophylline/Caffeine:** These are adenosine receptor antagonists; they can cause diuresis partly by inhibiting TGF-mediated afferent vasoconstriction.

Question 97: Which of the following statements regarding renal tubular functions is TRUE?

A. Fluid coming from the descending limb of the loop of Henle is hypotonic.
B. The descending limb of the loop of Henle is permeable to solutes.
C. If the clearance of a substance is greater than GFR, then tubular secretion must be present. (Correct Answer)
D. The clearance of a substance is always more than GFR if there is tubular secretion.

Explanation: ### Explanation **Correct Option: C (If the clearance of a substance is greater than GFR, then tubular secretion must be present.)** The Renal Clearance ($C_x$) of a substance is the volume of plasma cleared of that substance per unit time. The relationship between Clearance and GFR determines the net handling of a substance by the nephron: * **$C_x > GFR$:** This indicates that more of the substance is appearing in the urine than was filtered at the glomerulus. Therefore, the tubules must have **secreted** the substance into the lumen (e.g., Para-aminohippuric acid/PAH). * **$C_x < GFR$:** This indicates net **reabsorption** (e.g., Glucose, Urea). * **$C_x = GFR$:** This indicates the substance is neither secreted nor reabsorbed (e.g., Inulin). **Analysis of Incorrect Options:** * **Option A:** Fluid leaving the descending limb is **hypertonic**. As it descends into the hypermedullary interstitium, water is reabsorbed, concentrating the tubular fluid. * **Option B:** The descending limb is highly **permeable to water** but has low to **no permeability to solutes** (NaCl/Urea). This allows for the concentration of urine. * **Option D:** This is a common trap. If a substance is both reabsorbed and secreted (like Uric acid or Potassium), the *net* clearance depends on which process predominates. If reabsorption exceeds secretion, the clearance will still be less than the GFR despite secretion being present. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for GFR:** Inulin clearance (filtered only). * **Clinical Marker for GFR:** Creatinine clearance (slightly overestimates GFR because it is filtered + small amount secreted). * **Marker for Renal Plasma Flow (RPF):** PAH clearance (filtered + almost completely secreted). * **Site of maximum reabsorption:** Proximal Convoluted Tubule (PCT) reabsorbs ~65% of filtered load.

Question 98: Which of the following represents the volume of fluid filtered from the glomerulus into Bowman's capsule per unit time?

A. Glomerular filtration rate (Correct Answer)
B. Renal blood flow
C. Tubular maximum
D. Effective renal plasma flow

Explanation: ### Explanation **1. Why Glomerular Filtration Rate (GFR) is Correct:** The **Glomerular Filtration Rate (GFR)** is defined as the total volume of fluid filtered from the glomerular capillaries into the Bowman’s capsule per unit time (usually expressed in mL/min). It is the primary indicator of renal function. In a healthy adult, the average GFR is approximately **125 mL/min** or 180 L/day. It depends on the Starling forces (hydrostatic and oncotic pressures) across the glomerular membrane and the capillary filtration coefficient ($K_f$). **2. Why the Other Options are Incorrect:** * **Renal Blood Flow (RBF):** This represents the total volume of blood delivered to the kidneys per unit time (approx. 1100–1200 mL/min). Only a fraction of this blood (the plasma) is available for filtration. * **Tubular Maximum ($T_m$):** This refers to the maximum rate at which a specific substance (e.g., glucose) can be actively transported (reabsorbed or secreted) by the renal tubules. It is a measure of transport capacity, not filtration volume. * **Effective Renal Plasma Flow (ERPF):** This is the volume of plasma that reaches the functioning nephrons per unit time. It is typically measured using **Para-aminohippuric acid (PAH)** clearance and is approximately 600–700 mL/min. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Gold Standard Marker:** **Inulin clearance** is the gold standard for measuring GFR because it is freely filtered but neither reabsorbed nor secreted. * **Clinical Marker:** **Creatinine clearance** is the most common clinical method used to estimate GFR. * **Filtration Fraction (FF):** The ratio of GFR to Renal Plasma Flow (RPF). Normal FF is approximately **20%**, meaning 20% of the plasma entering the kidney is filtered into the Bowman's capsule. * **Formula:** $GFR = K_f \times [ (P_{gc} - P_{bc}) - (\pi_{gc} - \pi_{bc}) ]$ (Starling’s Equation).

Question 99: What is the normal Glomerular Filtration Rate (GFR)?

A. 125 ml/min (Correct Answer)
B. 90 ml/min
C. 60 ml/min
D. 150 ml/min

Explanation: **Explanation:** The **Glomerular Filtration Rate (GFR)** is the volume of fluid filtered from the renal glomerular capillaries into the Bowman’s capsule per unit of time. It is a key indicator of renal function. 1. **Why 125 ml/min is correct:** In a healthy adult male of average size (70 kg), the normal GFR is approximately **125 ml/min** (or 180 L/day). In females, it is slightly lower, around 110 ml/min. This value represents the sum of the filtration rates of all functioning nephrons in both kidneys. 2. **Why other options are incorrect:** * **90 ml/min:** While often considered the lower limit of "normal" in clinical practice, it is not the standard physiological baseline. A GFR below 90 ml/min typically indicates Stage 1 or 2 Chronic Kidney Disease (CKD) if other markers of kidney damage are present. * **60 ml/min:** This is a critical threshold. A GFR consistently below 60 ml/min for more than 3 months defines Chronic Kidney Disease (Stage 3). * **150 ml/min:** This value is higher than the physiological average and may be seen in states of "hyperfiltration," such as early-stage diabetic nephropathy or pregnancy. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Marker:** **Inulin clearance** is the gold standard for measuring GFR because it is freely filtered but neither reabsorbed nor secreted by the tubules. * **Clinical Marker:** **Creatinine clearance** is the most common clinical method used to estimate GFR, though it slightly overestimates GFR due to minor tubular secretion. * **Filtration Fraction:** Normal GFR (125) / Renal Plasma Flow (600) = **~20%**. This means 20% of the plasma entering the kidneys is filtered. * **Starling Forces:** GFR is determined by the balance of hydrostatic and oncotic pressures across the glomerular membrane.

Question 100: Endocrine functions associated with the kidneys include all of the following except?

A. Production of renin
B. Secretion of erythropoietin (Correct Answer)
C. Conversion of angiotensinogen to angiotensin I
D. Production of calcitriol

Explanation: The question asks for the option that is **NOT** an endocrine function of the kidney. While the kidney performs several endocrine tasks, the phrasing of the options requires a distinction between direct hormone production and enzymatic reactions. ### **Explanation of the Correct Answer** **Option B (Secretion of erythropoietin)** is marked as the "correct" answer in this specific MCQ context likely due to a technicality in how the question is framed or a potential error in the source key. **However, physiologically, the kidney DOES secrete erythropoietin (EPO).** In the context of NEET-PG, if this question is used to identify the "odd one out" regarding **enzymatic vs. hormonal** action: * **Renin (A)** is an enzyme produced by the Juxtaglomerular (JG) cells. * **Calcitriol (D)** is the active form of Vitamin D produced in the PCT. * **Angiotensinogen to Angiotensin I (C)** is a reaction catalyzed by Renin. *Note: If the question intended to ask which is NOT a function, Option C is technically the most accurate "non-endocrine" function because it is a biochemical conversion occurring in the plasma, not a hormone secreted by the kidney itself.* ### **Analysis of Other Options** * **A. Production of Renin:** Renin is an enzyme/hormone secreted by the JG cells of the afferent arteriole in response to low blood pressure or low sodium. * **C. Conversion of Angiotensinogen to Angiotensin I:** This is the primary function of Renin. While the conversion happens in the blood, it is the direct result of renal endocrine activity. * **D. Production of Calcitriol:** The kidneys contain the enzyme **1-alpha-hydroxylase**, which converts 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D (Calcitriol), the active form. ### **High-Yield NEET-PG Pearls** * **Erythropoietin (EPO):** 85% is produced by **peritubular interstitial fibroblasts** in the renal cortex. In chronic kidney disease (CKD), EPO deficiency leads to normocytic normochromic anemia. * **Renin:** Secreted by **JG cells** (modified smooth muscle cells). * **Prostaglandins:** The kidney also produces PGE2 and PGI2, which act as local vasodilators to maintain renal blood flow. * **Gluconeogenesis:** During prolonged fasting, the kidney is a major site of glucose production (up to 20%).

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

Practice Questions

Sodium and Water Balance

Practice Questions

Potassium Regulation

Practice Questions

Calcium and Phosphate Handling

Practice Questions

Micturition Physiology

Practice Questions

Renal Function Tests

Practice Questions

Integrative Responses to Fluid Challenges

Practice Questions

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