Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 211: Acute metabolic acidosis tends to _ intracellular K+ concentration and _ K+ secretion by the cortical collecting tubules?

A. Increase, increase
B. Increase, decrease
C. Decrease, increase
D. Decrease, decrease (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Decrease, decrease**. This phenomenon is rooted in the body’s mechanism to maintain pH balance at the expense of potassium homeostasis. **1. Why the correct answer is right:** * **Intracellular K+ Concentration:** In acute metabolic acidosis, there is a high concentration of $H^+$ ions in the extracellular fluid (ECF). To buffer this, $H^+$ ions move into the cells. To maintain electroneutrality, $K^+$ ions shift out of the cells into the ECF. This results in **decreased intracellular $K^+$ concentration** (and often hyperkalemia in the ECF). * **K+ Secretion:** In the Principal cells of the cortical collecting tubules, $K^+$ secretion depends on the activity of the $Na^+/K^+$ ATPase pump and the permeability of the apical membrane. Acute acidosis **inhibits the $Na^+/K^+$ ATPase pump**, reducing the uptake of $K^+$ from the blood into the cell. Furthermore, high $H^+$ levels reduce the permeability of the apical $K^+$ channels (ROMK). Both factors lead to **decreased $K^+$ secretion**. **2. Why incorrect options are wrong:** * **Options A & B:** These are incorrect because acidosis causes $K^+$ to leave the cell, not enter it; thus, intracellular $K^+$ cannot increase. * **Option C:** While intracellular $K^+$ does decrease, the second part is wrong because acidosis *inhibits* rather than increases secretion. Increased secretion is typically seen in alkalosis. **3. Clinical Pearls for NEET-PG:** * **Acute vs. Chronic:** While *acute* acidosis decreases $K^+$ secretion, *chronic* metabolic acidosis actually **increases** $K^+$ excretion. This is because chronic acidosis inhibits proximal tubule $NaCl$ and water reabsorption, leading to increased distal delivery of fluid, which washes away secreted $K^+$ and stimulates further secretion. * **The "Internal Balance" Rule:** For every 0.1 unit change in pH, the plasma $K^+$ concentration changes by approximately 0.6 mEq/L in the opposite direction. * **Aldosterone Paradox:** Acidosis directly inhibits $K^+$ secretion even if aldosterone levels are slightly elevated, as the direct effect of $pH$ on the $K^+$ channels is dominant in the acute phase.

Question 212: Maximum absorption of HCO3- occurs in which part of the nephron?

A. Proximal Convoluted Tubule (PCT) (Correct Answer)
B. Distal Convoluted Tubule (DCT)
C. Collecting Tubule (CT)
D. Ascending Limb of Loop of Henle (ALH)

Explanation: **Explanation:** The correct answer is **Proximal Convoluted Tubule (PCT)**. **1. Why PCT is the correct answer:** The PCT is the primary site for the reabsorption of the majority of filtered solutes. Approximately **80–90%** of filtered bicarbonate (HCO3-) is reabsorbed here. This process is mediated by the **Na+-H+ exchanger (NHE3)**, which secretes H+ into the lumen. The H+ combines with filtered HCO3- to form H2CO3, which is then broken down into CO2 and H2O by the enzyme **Carbonic Anhydrase (Type IV)** located on the brush border. CO2 diffuses into the cell, is converted back to HCO3-, and is transported into the blood via the Na+-HCO3- cotransporter (NBCe1). **2. Why other options are incorrect:** * **Ascending Limb of Loop of Henle (ALH):** Reabsorbs approximately **10–15%** of filtered bicarbonate. While significant, it is much less than the PCT. * **Distal Convoluted Tubule (DCT) & Collecting Tubule (CT):** These segments are responsible for the "fine-tuning" of acid-base balance. They reabsorb the remaining **~5%** of bicarbonate. The **Type A intercalated cells** in the collecting duct are crucial for secreting H+ and generating *new* bicarbonate during acidosis, but they do not handle the bulk of the filtered load. **3. NEET-PG High-Yield Clinical Pearls:** * **Carbonic Anhydrase Inhibitors (Acetazolamide):** Act primarily on the PCT. By inhibiting CA, they block HCO3- reabsorption, leading to alkaline urine and metabolic acidosis. * **Threshold for HCO3-:** The renal threshold for bicarbonate is approximately **24–26 mEq/L**. If plasma levels exceed this, HCO3- appears in the urine. * **Key Enzyme:** Remember that **Carbonic Anhydrase II** is intracellular, while **Carbonic Anhydrase IV** is membrane-bound (luminal).

Question 213: Injury to dorsal root nerve fibers in the sacral region of the spinal cord can cause which of the following conditions?

A. Atonic bladder (Correct Answer)
B. Neurogenic bladder
C. Automatic bladder
D. Any of the above

Explanation: **Explanation:** The micturition reflex is an autonomic reflex mediated by the sacral segments of the spinal cord (**S2, S3, and S4**). The sensory (afferent) limb of this reflex consists of fibers traveling via the pelvic nerves to the **dorsal roots** of the spinal cord. These fibers carry stretch signals from the bladder wall to the CNS. **1. Why Atonic Bladder is Correct:** When the **dorsal root nerve fibers (sensory limb)** are injured, the brain and spinal cord no longer receive signals indicating that the bladder is full. Consequently, the micturition reflex is never initiated. The bladder continues to fill beyond its capacity, leading to a thin-walled, non-contractile, and distended bladder. This is known as an **Atonic Bladder** (or Sensory Neurogenic Bladder). Clinical manifestation includes **overflow incontinence**, where urine dribbles out only when the intravesical pressure exceeds urethral resistance. **2. Why other options are incorrect:** * **Automatic Bladder (Spastic Bladder):** This occurs due to a spinal cord injury **above the sacral level**. The reflex arc remains intact, but voluntary control from the brain is lost. The bladder empties reflexively whenever it fills. * **Neurogenic Bladder:** This is a broad, non-specific umbrella term for any bladder dysfunction caused by neurologic damage. While technically correct in a general sense, "Atonic Bladder" is the specific physiological consequence of sensory (dorsal root) interruption. **High-Yield Clinical Pearls for NEET-PG:** * **Tabes Dorsalis:** A classic cause of atonic bladder due to syphilis-induced damage to the dorsal roots. * **De-efferented Bladder:** Caused by damage to the **motor (parasympathetic)** fibers; the bladder is also atonic. * **Autonomous Bladder:** Occurs when the **entire sacral reflex center** (conus medullaris) is destroyed; the bladder lacks both sensory and motor innervation.

Question 214: In the proximal convoluted tubule, H+ is exchanged for which ion?

A. K+
B. Na+ (Correct Answer)
C. HCO3-
D. Cl-

Explanation: **Explanation:** In the Proximal Convoluted Tubule (PCT), the secretion of Hydrogen ions ($H^+$) is primarily mediated by the **Sodium-Hydrogen Exchanger 3 (NHE3)**. This is a secondary active transport mechanism (antiport) where $Na^+$ moves down its electrochemical gradient into the cell, providing the energy to pump $H^+$ out into the tubular lumen. This process is crucial for bicarbonate ($HCO_3^-$) reabsorption and acid-base balance. **Analysis of Options:** * **B. Na+ (Correct):** The NHE3 antiporter specifically exchanges one luminal $Na^+$ for one intracellular $H^+$. This is the predominant mechanism for $H^+$ secretion in the early nephron. * **A. K+:** While $H^+/K^+$ exchange occurs in the **Alpha-Intercalated cells** of the late distal tubule and collecting duct (via $H^+/K^+$ ATPase), it is not the primary mechanism in the PCT. * **C. HCO3-:** $HCO_3^-$ is not exchanged for $H^+$; rather, secreted $H^+$ combines with filtered $HCO_3^-$ to form $H_2CO_3$, which is then broken down by carbonic anhydrase to facilitate $HCO_3^-$ reabsorption. * **D. Cl-:** $Cl^-$ is primarily reabsorbed in the PCT via paracellular pathways or in exchange for anions (like formate or oxalate), but not directly for $H^+$. **High-Yield Clinical Pearls for NEET-PG:** * **Carbonic Anhydrase (CA):** CA-IV (luminal) and CA-II (intracellular) are essential for the PCT to reabsorb ~80% of filtered bicarbonate. * **Acetazolamide:** A diuretic that inhibits CA, leading to decreased $H^+$ secretion and $HCO_3^-$ loss, resulting in **Proximal Renal Tubular Acidosis (Type 2 RTA)**. * **Angiotensin II:** Stimulates the NHE3 exchanger in the PCT, increasing $Na^+$ reabsorption and $H^+$ secretion (contributing to contraction alkalosis).

Question 215: All of the following statements about angiotensin II are TRUE, EXCEPT:

A. Autoregulation of GFR
B. Release aldosterone
C. Secreted from endothelium (Correct Answer)
D. Constriction of afferent arteriole

Explanation: **Explanation:** The correct answer is **C (Secreted from endothelium)**. This statement is false because Angiotensin II is not "secreted" by the endothelium; rather, it is **formed in the pulmonary and renal circulation** via the action of Angiotensin-Converting Enzyme (ACE) on Angiotensin I. While ACE is located on the surface of vascular endothelial cells, the precursor (Angiotensinogen) is secreted by the liver and converted to Angiotensin I by Renin (secreted by the JGA). **Analysis of other options:** * **A. Autoregulation of GFR:** Angiotensin II plays a critical role in maintaining GFR during states of low renal perfusion pressure. By preferentially constricting the efferent arteriole, it increases glomerular hydrostatic pressure, thereby stabilizing GFR. * **B. Release aldosterone:** Angiotensin II acts directly on the **Zona Glomerulosa** of the adrenal cortex to stimulate the synthesis and secretion of aldosterone, which promotes sodium and water retention. * **D. Constriction of afferent arteriole:** While Angiotensin II is more potent at the efferent arteriole, in high concentrations (pathological states or severe hypovolemia), it also causes constriction of the **afferent arteriole**, leading to a decrease in both Renal Blood Flow (RBF) and GFR. **High-Yield Clinical Pearls for NEET-PG:** * **ACE Inhibitors/ARBs:** These drugs are contraindicated in **Bilateral Renal Artery Stenosis** because they block Angiotensin II-mediated efferent vasoconstriction, leading to a precipitous drop in GFR and acute renal failure. * **Potency:** Angiotensin II is one of the most potent vasoconstrictors in the body. * **Thirst:** It also acts on the **Subfornical Organ** in the brain to stimulate the thirst center and ADH release.

Question 216: Pressure diuresis lowers arterial pressure because it?

A. Lowers blood volume (Correct Answer)
B. Lowers renal release of renin
C. Lowers systemic vascular resistance
D. Causes renal vasodilation

Explanation: **Explanation:** **Pressure Diuresis** is a key mechanism of the renal-body fluid feedback system for long-term arterial pressure control. 1. **Why Option A is correct:** When arterial pressure rises, the kidneys increase the excretion of water (**pressure diuresis**) and sodium (**pressure natriuresis**). This occurs because the increased pressure directly increases the glomerular filtration rate (GFR) and inhibits tubular reabsorption of water and sodium. The resulting loss of fluid **lowers the extracellular fluid volume and blood volume**, which reduces venous return and cardiac output, ultimately bringing the arterial pressure back toward the normal set point. 2. **Why other options are incorrect:** * **Option B:** While high pressure does eventually suppress renin release, the primary physical mechanism by which pressure diuresis *lowers* pressure is the direct reduction of fluid volume. * **Option C:** Pressure diuresis affects the "volume" component of the blood pressure equation ($BP = CO \times SVR$), not the systemic vascular resistance directly. * **Option D:** Renal vasodilation may occur as an autoregulatory response to high pressure, but it is a *cause* or a concomitant finding, not the mechanism by which diuresis lowers systemic arterial pressure. **High-Yield Clinical Pearls for NEET-PG:** * **Guyton’s Curve:** The relationship between arterial pressure and urinary output is nearly linear; a slight increase in BP can cause a 2-3 fold increase in urine output. * **Pressure Natriuresis:** This refers specifically to the increased excretion of sodium in response to elevated pressure; it usually occurs alongside pressure diuresis. * **Chronic vs. Acute:** The renal-body fluid mechanism is the most potent **long-term** regulator of blood pressure, unlike the baroreceptor reflex, which is for short-term/acute changes.

Question 217: What is the normal range of urine osmolality (mosmol/L)?

A. 50 - 1400
B. 100 - 1400
C. 200 - 1000
D. 300 - 1200 (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (300 - 1200 mosmol/L)**. **1. Understanding the Concept:** Urine osmolality is a measure of the concentration of dissolved particles in the urine and reflects the kidney's ability to concentrate or dilute urine based on the body's hydration status. While the human kidney has a remarkable physiological range (from a minimum of ~50 to a maximum of ~1200–1400 mOsm/L), the **standard clinical "normal range"** for a healthy adult with average fluid intake is typically cited as **300 to 1200 mOsm/L**. * **300 mOsm/L** represents isotonicity (equal to plasma). * **1200 mOsm/L** represents the maximum concentrating capacity of the juxtamedullary nephrons via the countercurrent multiplier system. **2. Analysis of Incorrect Options:** * **A & B (50 - 1400):** These represent the **extreme physiological limits** of the kidney under conditions of maximum water loading or severe dehydration/ADH stimulation. While technically possible, they do not represent the "normal range" seen in routine clinical practice. * **C (200 - 1000):** This range is too narrow and underestimates the kidney's ability to concentrate urine to the level of the deep medullary osmotic gradient (1200 mOsm/L). **3. NEET-PG High-Yield Pearls:** * **Specific Gravity vs. Osmolality:** Osmolality is a more accurate reflection of renal concentrating ability than specific gravity because it is not affected by the size or weight of molecules (like glucose or protein). * **Fixed Specific Gravity (Isosthenuria):** A constant urine osmolality of ~300 mOsm/L (matching plasma) regardless of water intake is a hallmark of **Chronic Renal Failure**, indicating the loss of both concentrating and diluting functions. * **ADH Role:** The maximum concentration of 1200 mOsm/L is entirely dependent on **Antidiuretic Hormone (ADH)** acting on the V2 receptors in the collecting ducts.

Question 218: The anemia in chronic renal failure (CRF) is primarily due to which pathophysiological mechanism?

A. Decreased erythropoiesis (Correct Answer)
B. Increased hemolysis
C. Both decreased erythropoiesis and increased hemolysis
D. None of the above

Explanation: **Explanation:** The primary cause of anemia in Chronic Renal Failure (CRF) is a deficiency in **Erythropoietin (EPO)** production. Erythropoietin is a glycoprotein hormone synthesized by the **peritubular interstitial cells** of the renal cortex in response to hypoxia. In CRF, the progressive destruction of renal parenchyma leads to a significant decline in EPO levels, resulting in **decreased erythropoiesis** (reduced red blood cell production) in the bone marrow. This typically manifests as a normocytic, normochromic anemia. **Analysis of Options:** * **Option A (Correct):** As explained, the loss of functional renal tissue directly impairs the synthesis of EPO, the primary stimulator of the bone marrow to produce RBCs. This is the hallmark pathophysiological mechanism. * **Option B (Incorrect):** While the uremic environment in advanced CRF can slightly shorten the lifespan of RBCs (mild hemolysis), it is a secondary factor and not the *primary* cause of the anemia. * **Option C (Incorrect):** Although both factors can exist, "decreased erythropoiesis" is the definitive primary mechanism. In the context of NEET-PG, when asked for the "primary" or "most important" cause, EPO deficiency (decreased production) is the prioritized answer. **High-Yield Clinical Pearls for NEET-PG:** * **Target Hb:** In CRF patients on EPO therapy, the target Hemoglobin is usually **10–11 g/dL**. Higher levels (above 13 g/dL) are avoided due to increased risk of cardiovascular events and stroke. * **Iron Stores:** Before starting EPO, always check iron stores (Ferritin/TSAT), as EPO therapy is ineffective in the presence of iron deficiency. * **Other contributing factors in CRF:** Chronic blood loss (due to platelet dysfunction/uremic gastropathy) and secondary hyperparathyroidism (causing marrow fibrosis).

Question 219: Which of the following is NOT an action of the kidney?

A. Erythropoietin secretion
B. Regulation of 1,25-dihydroxyvitamin D3 production
C. Glycogenesis (Correct Answer)
D. Regulation of water and electrolyte balance

Explanation: The kidney is a multifunctional organ responsible for excretory, homeostatic, and endocrine activities. **Explanation of the Correct Answer:** **C. Glycogenesis:** This is the process of synthesizing glycogen from glucose for storage. While the kidney is capable of **Gluconeogenesis** (synthesizing glucose from non-carbohydrate sources like amino acids, especially during prolonged fasting), it does not perform glycogenesis to any significant degree. Glycogen storage is primarily a function of the **liver and skeletal muscles**. **Why the other options are incorrect (Functions of the Kidney):** * **A. Erythropoietin secretion:** The kidney (specifically the interstitial cells in the peritubular capillary bed) produces erythropoietin in response to hypoxia, which stimulates RBC production in the bone marrow. * **B. Regulation of 1,25-dihydroxyvitamin D3:** The kidney contains the enzyme **1-alpha-hydroxylase**, which converts 25-hydroxyvitamin D into its active form, Calcitriol (1,25-dihydroxycholecalciferol). * **D. Regulation of water and electrolyte balance:** This is the primary homeostatic function of the kidney, achieved through glomerular filtration, tubular reabsorption, and secretion (regulated by hormones like ADH and Aldosterone). **High-Yield Clinical Pearls for NEET-PG:** * **Gluconeogenesis:** During starvation, the kidney can contribute up to **20-25%** of the body's glucose production. * **Hormones secreted by Kidney:** Renin (by JG cells), Erythropoietin, and Calcitriol. * **Chronic Kidney Disease (CKD):** Patients often present with **anemia** (due to decreased Erythropoietin) and **renal osteodystrophy** (due to failure of Vitamin D activation).

Question 220: What substances are involved in the countercurrent mechanism for maintaining the medullary osmotic gradient?

A. Sodium chloride (NaCl)
B. Urea
C. Sodium chloride (NaCl) and Urea
D. Sodium chloride (NaCl), Urea, and Water (Correct Answer)

Explanation: ### Explanation The **medullary osmotic gradient** (ranging from 300 mOsm/L at the cortex to 1200 mOsm/L at the papilla) is essential for urine concentration. This gradient is established and maintained by the **Countercurrent Mechanism**, which involves three critical components: 1. **Sodium Chloride (NaCl):** Primarily contributed by the **Countercurrent Multiplier** (Loop of Henle). The thick ascending limb actively reabsorbs Na⁺ and Cl⁻ into the medullary interstitium, accounting for approximately **50%** of the gradient. 2. **Urea:** Contributed by **Urea Recycling**. Urea moves out of the medullary collecting ducts into the deep medullary interstitium, accounting for the remaining **40-50%** of the osmolarity. 3. **Water:** Managed by the **Countercurrent Exchanger** (Vasa Recta). The vasa recta removes excess water reabsorbed from the descending limb and collecting ducts. Without this continuous removal of water, the medullary interstitium would become diluted, and the osmotic gradient would be "washed out." #### Analysis of Options: * **Options A & B:** These are incomplete. While NaCl and Urea are the primary solutes, they cannot maintain the gradient in isolation without the fluid dynamics of water. * **Option C:** Incorrect because it ignores the role of the vasa recta in preventing the dissipation of the gradient by water. * **Option D (Correct):** Accurately identifies that the gradient is a balance between solute deposition (NaCl, Urea) and the removal of reabsorbed water. #### High-Yield Clinical Pearls for NEET-PG: * **Vasa Recta:** Acts as an exchanger; its slow blood flow is crucial to prevent "solute washout." * **ADH (Vasopressin):** Increases the gradient by increasing the permeability of the medullary collecting duct to urea (via UT-A1 transporters). * **Protein Malnutrition:** Leads to a decreased ability to concentrate urine because of reduced urea levels, impairing the medullary gradient.

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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