Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance Indian Medical PG Practice Questions and MCQs

Question 141: Which statement is not true regarding hypocalcemia?

A. Can occur in hypoparathyroidism
B. Latent tetany is seen
C. Prolonged QT interval
D. Inverse relation with magnesium levels (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**, as hypocalcemia actually has a **direct (positive) relationship** with magnesium levels, not an inverse one. **1. Why Option D is the correct answer (The False Statement):** Magnesium is essential for the synthesis and release of Parathyroid Hormone (PTH) and for the responsiveness of target organs to PTH. Therefore, **hypomagnesemia** leads to functional hypoparathyroidism, resulting in **hypocalcemia**. Correcting magnesium levels is often necessary to resolve refractory hypocalcemia. **2. Analysis of Incorrect Options (True Statements):** * **A. Hypoparathyroidism:** PTH is the primary hormone that increases serum calcium by mobilizing it from bones and increasing renal reabsorption. A deficiency in PTH directly leads to low serum calcium levels. * **B. Latent Tetany:** Hypocalcemia increases neuromuscular excitability. Latent tetany refers to signs that are not clinically apparent but can be elicited, such as **Chvostek’s sign** (facial twitching on tapping the facial nerve) and **Trousseau’s sign** (carpedal spasm on inflating a BP cuff). * **C. Prolonged QT Interval:** On an ECG, hypocalcemia slows the phase 2 of the cardiac action potential, leading to a characteristic prolongation of the ST segment and the **QT interval**. **Clinical Pearls for NEET-PG:** * **Hypercalcemia** causes a **shortened** QT interval (mnemonic: "Short Cow" for Short QT/Calcium). * **Pseudohypocalcemia:** Always check serum albumin. For every 1 g/dL drop in albumin below 4 g/dL, add 0.8 mg/dL to the measured calcium. * **Hyperphosphatemia** (as seen in Chronic Kidney Disease) has an **inverse** relationship with calcium.

Question 142: Excessive vomiting with gastric outlet obstruction secondary to pyloric stenosis leads to which of the following acid-base and electrolyte disturbances?

A. Hypokalemic hypochloremic metabolic alkalosis (Correct Answer)
B. Hypokalemic hyperchloremic metabolic acidosis
C. Hyperkalemic hyperchloremic metabolic alkalosis
D. Hyperkalemic hypochloremic metabolic acidosis

Explanation: ### Explanation **1. Why Option A is Correct:** Gastric outlet obstruction (e.g., pyloric stenosis) results in persistent vomiting of gastric contents, which are rich in **Hydrochloric acid (HCl)** and **Potassium (K⁺)**. * **Metabolic Alkalosis:** Loss of H⁺ ions from the stomach leads to a rise in plasma bicarbonate ($HCO_3^-$). * **Hypochloremia:** Direct loss of Chloride (Cl⁻) in the vomitus. * **Hypokalemia:** This occurs via two mechanisms: 1. Direct loss in gastric juice. 2. **Renal compensation:** To conserve Na⁺ in the face of dehydration, the kidneys activate the Renin-Angiotensin-Aldosterone System (RAAS). Aldosterone causes Na⁺ reabsorption at the expense of K⁺ and H⁺ excretion in the distal tubule. * **Paradoxical Aciduria:** In severe cases, the kidney prioritizes Na⁺ conservation over pH balance. It excretes H⁺ ions instead of K⁺ to save Na⁺, leading to acidic urine despite systemic alkalosis. **2. Why Other Options are Incorrect:** * **Options B & D (Acidosis):** Vomiting causes a loss of acid (H⁺), not a gain. Metabolic acidosis is typically seen in lower GI losses (diarrhea), where bicarbonate is lost. * **Option C (Hyperkalemia):** Alkalosis generally causes a shift of K⁺ into cells (intracellular shift) and increased renal excretion, leading to hypokalemia, never hyperkalemia. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Classic" Picture:** Pyloric stenosis presents with non-bilious, projectile vomiting and a palpable "olive-shaped" mass. * **Urine Findings:** Initially, urine is alkaline (due to $HCO_3^-$ excretion), but as dehydration worsens, it becomes acidic (**Paradoxical Aciduria**). * **Treatment Priority:** The first step in management is fluid resuscitation with **0.9% Normal Saline** (to provide Cl⁻ and volume) and Potassium supplementation, *before* surgical correction (Ramstedt’s Pyloromyotomy).

Question 143: Which of the following conditions are associated with hypophosphatemia?

A. Pseudohypoparathyroidism
B. Renal tubular acidosis (Correct Answer)
C. Rickets
D. Respiratory acidosis

Explanation: **Explanation:** Hypophosphatemia occurs when there is a shift of phosphate into cells, decreased intestinal absorption, or increased renal excretion. **Why Renal Tubular Acidosis (RTA) is correct:** In both Type 1 (Distal) and Type 2 (Proximal) RTA, there is significant renal phosphate wasting. In **Type 2 RTA (Fanconi Syndrome)**, the proximal tubule fails to reabsorb solutes, directly leading to phosphaturia. In **Type 1 RTA**, the chronic metabolic acidosis causes bone buffering, which releases calcium and phosphate; the resulting hypercalciuria leads to secondary hyperparathyroidism, which further increases renal phosphate excretion, causing hypophosphatemia. **Analysis of Incorrect Options:** * **Pseudohypoparathyroidism:** This is characterized by end-organ resistance to PTH. Since PTH normally promotes phosphate excretion, resistance leads to **hyperphosphatemia** (similar to hypoparathyroidism). * **Rickets:** While Vitamin D deficiency rickets is classically associated with hypophosphatemia, the question asks for the *most* definitive association among options. In the context of NEET-PG, RTA is a high-yield cause of profound renal phosphate loss. (Note: If this were a multiple-choice "select all," Rickets would apply, but RTA is the primary metabolic focus here). * **Respiratory Acidosis:** Acidosis generally causes phosphate to shift **out** of cells into the extracellular fluid, potentially causing hyperphosphatemia. Conversely, respiratory *alkalosis* is a major cause of hypophosphatemia. **High-Yield Clinical Pearls for NEET-PG:** * **Refeeding Syndrome:** The most common cause of severe hypophosphatemia in hospitalized patients (due to insulin-mediated intracellular shift). * **PTH Effect:** PTH is "Phosphaturic" (decreases reabsorption in the proximal tubule). * **Fanconi Syndrome Triad:** Phosphaturia (hypophosphatemia), Glycosuria (with normal blood glucose), and Aminoaciduria.

Question 144: The presence of two extra pairs of electrons on the oxygen atom in a water molecule results in what?

A. Water behaving as a non-polar solvent
B. Formation of covalent bonds in ice
C. An electronegative charge on the water molecule (Correct Answer)
D. An electropositive charge on the water molecule

Explanation: **Explanation:** The water molecule ($H_2O$) is characterized by a **bent geometry** due to the $sp^3$ hybridization of the oxygen atom. Oxygen has six valence electrons: two are shared in covalent bonds with hydrogen atoms, while the remaining four form **two pairs of unshared electrons (lone pairs)**. **Why Option C is Correct:** Oxygen is significantly more **electronegative** than hydrogen. This means it has a stronger affinity for electrons. The presence of these two lone pairs, combined with oxygen's ability to pull shared electrons away from the hydrogen atoms, creates an asymmetric distribution of charge. This results in a **partial negative charge ($\delta^-$)** near the oxygen atom and a partial positive charge ($\delta^+$) near the hydrogen atoms, making the molecule a **dipole**. **Analysis of Incorrect Options:** * **Option A:** Water is the "universal **polar** solvent." Its polarity allows it to dissolve electrolytes and polar organic molecules by forming hydration shells. * **Option B:** While ice involves a rigid lattice, this structure is maintained by **Hydrogen bonds**, not covalent bonds (which exist within the molecule itself). * **Option D:** The oxygen atom carries the electronegative charge; the electropositive charge is localized on the hydrogen atoms. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hydrogen Bonding:** The dipolar nature allows water to form hydrogen bonds, which are responsible for its high specific heat and surface tension—crucial for thermoregulation in humans. * **Solubility:** "Like dissolves like." Water’s polarity is why hydrophobic (non-polar) molecules like lipids require transport proteins (e.g., albumin, lipoproteins) in the blood. * **Amphoteric Nature:** Water can act as both an acid and a base, a fundamental property for maintaining the body's acid-base equilibrium.

Question 145: Which of the following causes normal anion gap metabolic acidosis?

A. Cholera (Correct Answer)
B. Starvation
C. Ethylene glycol poisoning
D. Lactic acidosis

Explanation: **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. **Why Cholera is correct:** Cholera causes severe secretory diarrhea. In the lower gastrointestinal tract, secretions are rich in bicarbonate ($HCO_3^-$). The direct loss of bicarbonate leads to a decrease in the blood pH. To maintain electroneutrality, the kidneys retain Chloride ($Cl^-$) ions. Because the decrease in $HCO_3^-$ is offset by an increase in $Cl^-$, the total anion gap remains within the normal range (8–12 mEq/L). This is why it is also termed **Hyperchloremic Metabolic Acidosis**. **Why the other options are incorrect:** * **B, C, and D (Starvation, Ethylene glycol, Lactic acidosis):** These are all causes of **High Anion Gap Metabolic Acidosis (HAGMA)**. In these conditions, metabolic acidosis occurs due to the accumulation of unmeasured organic acids (Ketoacids in starvation, Glycolic acid in ethylene glycol, and Lactic acid in hypoxia). These acid anions "replace" bicarbonate without a corresponding increase in chloride, thus widening the gap. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for NAGMA (Normal Anion Gap):** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra-chloride, Diarrhea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Key Distinction:** Diarrhea (NAGMA) vs. Vomiting (Metabolic Alkalosis).

Question 146: All of the following are seen in chronic pyloric obstruction except?

A. Alkaline urine (Correct Answer)
B. Acidic urine
C. Hypochloremia
D. Hypokalemia

Explanation: In chronic pyloric obstruction (e.g., Gastric Outflow Obstruction), repeated vomiting leads to a classic metabolic derangement known as **Metabolic Alkalosis with Paradoxical Aciduria**. ### **Why "Alkaline Urine" is the Correct Answer (The "Except")** In the early stages, the urine is alkaline due to bicarbonate excretion. However, in **chronic** obstruction, the body develops **Paradoxical Aciduria**. Despite being in a state of systemic alkalosis, the kidneys excrete acidic urine. This happens because: 1. **Volume Depletion:** Activates the Renin-Angiotensin-Aldosterone System (RAAS). 2. **Sodium Conservation:** To save water, the kidney reabsorbs $Na^+$. To maintain electrical neutrality, it must excrete either $K^+$ or $H^+$. 3. **Potassium Depletion:** As $K^+$ stores are depleted, the kidney is forced to exchange $Na^+$ for $H^+$ ions, leading to acidic urine. ### **Explanation of Other Options** * **Hypochloremia:** Vomitus contains high amounts of $HCl$. Loss of chloride leads to hypochloremic metabolic alkalosis. * **Hypokalemia:** Occurs due to direct loss in vomitus and, more significantly, due to renal wasting (exchange for $Na^+$) and intracellular shifts. * **Acidic Urine:** As explained above, this is the hallmark of the chronic phase (Paradoxical Aciduria). ### **NEET-PG High-Yield Pearls** * **The Triad:** Hypochloremic, hypokalemic, metabolic alkalosis. * **Paradoxical Aciduria:** Occurs when the kidney prioritizes volume expansion (via $Na^+$ reabsorption) over pH balance. * **Treatment:** The most important initial step is **Normal Saline (0.9% NaCl)**. Chloride replacement allows the kidney to stop excreting $H^+$ and start excreting $HCO_3^-$, correcting the alkalosis.

Question 147: Thyroid peroxidase is not involved in which of the following processes?

A. Oxidation of iodide to atomic iodine
B. Acting as a frequent epitope for autoantibodies
C. Secretion of thyroglobulin into the colloid (Correct Answer)
D. Liberation of iodine for addition onto tyrosine residues on thyroglobulin

Explanation: **Explanation:** Thyroid Peroxidase (TPO) is a membrane-bound enzyme located on the apical surface of thyroid follicular cells. It is the central enzyme in thyroid hormone synthesis, but it does not participate in the transport or secretion of proteins. **Why Option C is correct:** The secretion of **thyroglobulin (Tg)** into the follicular colloid is a process of **exocytosis**. Tg is synthesized in the rough endoplasmic reticulum, packaged in the Golgi apparatus, and transported via vesicles to the apical membrane. This is a cellular transport mechanism independent of TPO enzymatic activity. **Why the other options are incorrect:** * **Option A:** TPO uses hydrogen peroxide ($H_2O_2$) to catalyze the **oxidation** of iodide ($I^-$) into an active form (atomic iodine or $I^0$), which is essential for the next steps. * **Option B:** TPO is a major **autoantigen**. Antibodies against TPO (Anti-TPO) are the hallmark of **Hashimoto’s thyroiditis** and are frequently used in clinical diagnosis. * **Option D:** TPO facilitates **organification**, where the activated iodine is added to tyrosine residues on the thyroglobulin backbone to form Monoiodotyrosine (MIT) and Diiodotyrosine (DIT). **High-Yield NEET-PG Pearls:** 1. **Wolff-Chaikoff Effect:** High levels of iodine transiently inhibit TPO, leading to a decrease in thyroid hormone synthesis. 2. **Pendred Syndrome:** Caused by a mutation in the *SLC26A4* gene (Pendrin transporter), leading to defective iodide transport into the colloid and sensorineural hearing loss. 3. **Inhibition:** Thionamides (Propylthiouracil and Methimazole) act by inhibiting TPO, making them the mainstay treatment for hyperthyroidism. 4. **Coupling:** TPO also catalyzes the coupling of MIT/DIT to form $T_3$ and $T_4$.

Question 148: What is the normal serum potassium level?

A. Below 3.5 mEq/L (Correct Answer)
B. Below 4.5 mEq/L
C. Below 5.6 mEq/L
D. Below 6.5 mEq/L

Explanation: **Explanation:** Potassium ($K^+$) is the primary intracellular cation, with approximately 98% of the body's potassium located inside cells. The normal serum potassium range is typically **3.5 to 5.0 mEq/L**. **Why Option A is correct:** In the context of the question asking for the threshold of "normal" levels, **3.5 mEq/L** represents the lower limit of the physiological range. Therefore, a value falling **below 3.5 mEq/L** is clinically defined as **hypokalemia**. Maintaining this narrow range is critical for the resting membrane potential of excitable tissues, particularly the myocardium. **Analysis of Incorrect Options:** * **Option B (4.5 mEq/L):** This is the midpoint of the normal range. Values below 4.5 but above 3.5 are considered normal, not pathological. * **Option C (5.6 mEq/L):** This value exceeds the upper limit (5.0–5.5 mEq/L) and indicates **hyperkalemia**. * **Option D (6.5 mEq/L):** This represents **severe hyperkalemia**, a medical emergency associated with a high risk of cardiac arrest (sine wave pattern on ECG). **High-Yield Clinical Pearls for NEET-PG:** 1. **ECG Changes in Hypokalemia (<3.5):** Flattening/Inversion of T-waves, prominent **U-waves**, and ST-segment depression. 2. **ECG Changes in Hyperkalemia (>5.5):** **Tall peaked T-waves**, prolonged PR interval, and widening of the QRS complex. 3. **Insulin and Alkalosis:** Both shift potassium from the extracellular fluid (ECF) into the cells, potentially causing hypokalemia. 4. **Aldosterone:** The primary hormone regulating potassium excretion via the principal cells of the distal convoluted tubule and collecting duct.

Question 149: Which of the following is NOT a cause of high anion gap metabolic acidosis?

A. Lactic acidosis
B. Diabetic ketoacidosis
C. Renal failure
D. Diarrhea (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. A high anion gap occurs when "unmeasured anions" (like lactate or ketones) accumulate, consuming bicarbonate. **Why Diarrhea is the Correct Answer:** Diarrhea is a classic cause of **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic metabolic acidosis. In diarrhea, there is a direct gastrointestinal loss of bicarbonate ($HCO_3^-$). To maintain electroneutrality, the kidneys retain Chloride ($Cl^-$). Since the decrease in bicarbonate is balanced by an increase in chloride, the total anion gap remains within the normal range (8–12 mEq/L). **Why the other options are incorrect:** * **Lactic Acidosis:** Accumulation of lactate (unmeasured anion) increases the gap. Common in shock or sepsis. * **Diabetic Ketoacidosis (DKA):** Accumulation of acetoacetate and $\beta$-hydroxybutyrate (unmeasured anions) leads to a high gap. * **Renal Failure:** In advanced chronic kidney disease, the kidneys fail to excrete organic acids (phosphates, sulfates), which function as unmeasured anions. **NEET-PG High-Yield Pearls:** * **Mnemonic for High AG Acidosis:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for Normal AG Acidosis (NAGMA):** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Golden Rule:** If the question mentions **bicarbonate loss** (Diarrhea or RTA), think **Normal AG**. If it mentions **acid addition**, think **High AG**.

Question 150: Metabolic changes associated with excessive vomiting include which of the following?

A. Metabolic acidosis
B. Hyperchloremia
C. Hypokalemia (Correct Answer)
D. Decreased bicarbonates

Explanation: **Explanation:** Excessive vomiting leads to a classic acid-base disturbance known as **Metabolic Alkalosis with Hypokalemia and Hypochloremia.** **Why Hypokalemia is correct:** Hypokalemia occurs due to three primary mechanisms: 1. **Direct Loss:** Gastric juice contains small amounts of potassium. 2. **Renal Compensation:** To compensate for the loss of H+ ions, the kidneys attempt to conserve H+ at the expense of excreting K+ in the distal tubule. 3. **Secondary Hyperaldosteronism:** Vomiting causes volume depletion (dehydration), which activates the Renin-Angiotensin-Aldosterone System (RAAS). Aldosterone acts on the kidneys to reabsorb Na+ and water, while further increasing the secretion of K+ into the urine. **Analysis of Incorrect Options:** * **A & D (Metabolic Acidosis/Decreased Bicarbonates):** Vomiting causes a loss of hydrochloric acid (HCl) from the stomach. The loss of H+ ions leads to an increase in plasma bicarbonate ($HCO_3^-$), resulting in **Metabolic Alkalosis**, not acidosis. * **B (Hyperchloremia):** Gastric juice is rich in chloride. Excessive vomiting leads to significant chloride loss, resulting in **Hypochloremia**. **NEET-PG High-Yield Pearls:** * **Paradoxical Aciduria:** In severe vomiting, despite systemic alkalosis, the urine becomes acidic. This happens because the body prioritizes volume expansion (Na+ reabsorption) over pH balance; to reabsorb Na+ when K+ is already depleted, the kidney is forced to secrete H+ ions. * **Classic Presentation:** A child with Hypertrophic Pyloric Stenosis typically presents with "Hypochloremic, Hypokalemic, Metabolic Alkalosis with Paradoxical Aciduria."

Practice by Chapter

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pH Regulation in Body Fluids

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Respiratory Regulation of Acid-Base Balance

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Renal Regulation of Acid-Base Balance

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Respiratory and Metabolic Acidosis

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Respiratory and Metabolic Alkalosis

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Mixed Acid-Base Disorders

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Interpretation of Arterial Blood Gases

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Electrolyte Homeostasis

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Sodium and Water Balance

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Potassium Balance

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Calcium and Phosphate Metabolism

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