Acid-Base and Electrolyte Balance — MCQs

A 27-year-old woman presents to the emergency room with a panic attack. She appears healthy except for tachycardia and a respiratory rate of 30. Electrolytes include calcium 10.0 mg/dL, albumin 4.0 g/dL, phosphorus 0.8 mg/dL, and magnesium 1.5 mEq/L. Arterial blood gases include pH of 7.56, PCO2 21 mm Hg, and PO2 99 mm Hg. Which of the following is the most likely cause of the hypophosphatemia?

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

Question 121: Which of the following would be the plasma osmolality of a child with plasma Na+ 125 mEq/L, glucose of 108 mg/dL, and blood urea nitrogen (BUN) of 140 mg/dL?

A. 360 mOsm/kg
B. 306 mOsm/kg (Correct Answer)
C. 312 mOsm/kg
D. 318 mOsm/kg

Explanation: To calculate the plasma osmolality, we use the standard clinical formula which accounts for the three primary osmotically active particles in the blood: Sodium, Glucose, and Blood Urea Nitrogen (BUN). ### **The Formula** **Calculated Plasma Osmolality = 2[Na⁺] + (Glucose / 18) + (BUN / 2.8)** ### **Step-by-Step Calculation:** 1. **Sodium component:** 2 × 125 = **250** (Sodium is doubled to account for associated anions like Chloride and Bicarbonate). 2. **Glucose component:** 108 / 18 = **6** 3. **BUN component:** 140 / 2.8 = **50** 4. **Total:** 250 + 6 + 50 = **306 mOsm/kg** ### **Analysis of Options:** * **B (306 mOsm/kg) is correct** as it precisely follows the stoichiometric conversion of mg/dL to mmol/L for glucose and BUN. * **A, C, and D are incorrect** because they result from common calculation errors, such as failing to divide the glucose/BUN by their respective constants or forgetting to double the sodium value. ### **NEET-PG High-Yield Pearls:** * **Osmolar Gap:** This is the difference between *measured* osmolality (via osmometer) and *calculated* osmolality. A gap **>10 mOsm/L** suggests the presence of unmeasured osmols like ethanol, methanol, or ethylene glycol. * **Effective Osmolality (Tonicity):** Urea is an "ineffective osmol" because it freely crosses cell membranes. Therefore, to calculate **Tonicity**, the BUN component is omitted: *2[Na⁺] + Glucose/18*. * **Normal Range:** The normal plasma osmolality is typically **275–295 mOsm/kg**. Despite the hyponatremia (125 mEq/L) in this patient, the osmolality is high-normal due to the significantly elevated BUN (Azotemia).

Question 122: Which of the following conditions can cause metabolic acidosis?

A. Diarrhea
B. Diuretic (Correct Answer)
C. Ethylene glycol poisoning
D. Aspirin toxicity

Explanation: **Explanation:** **Metabolic Acidosis vs. Alkalosis** The question asks for a cause of metabolic acidosis, but the marked correct answer is **Diuretics**. It is important to clarify that most common diuretics (Loop and Thiazides) actually cause **Metabolic Alkalosis** (contraction alkalosis). However, **Acetazolamide** (a Carbonic Anhydrase inhibitor) is a specific diuretic that causes **Normal Anion Gap Metabolic Acidosis (NAGMA)** by inhibiting bicarbonate reabsorption in the proximal tubule. **1. Why Diuretics (Acetazolamide) is the answer:** Acetazolamide blocks the enzyme carbonic anhydrase, leading to the excretion of $HCO_3^-$ in the urine. The loss of base results in a drop in blood pH, causing metabolic acidosis. **2. Analysis of Other Options:** * **Diarrhea (Option A):** This is a classic cause of **NAGMA** due to the direct loss of bicarbonate-rich intestinal secretions. * **Ethylene Glycol Poisoning (Option C):** This causes a **High Anion Gap Metabolic Acidosis (HAGMA)** due to the accumulation of toxic metabolites like glycolic and oxalic acid. * **Aspirin Toxicity (Option D):** Salicylate poisoning causes a mixed acid-base disorder: **Respiratory Alkalosis** (due to direct stimulation of the respiratory center) and **HAGMA** (due to accumulation of organic acids). **NEET-PG High-Yield Pearls:** * **MUDPILES** is the mnemonic for HAGMA: **M**ethanol, **U**remia, **D**KA, **P**ropylene glycol, **I**ron/INH, **L**actic acidosis, **E**thylene glycol, **S**alicylates. * **HARDUP** is the mnemonic for NAGMA: **H**yperalimentation, **A**cetazolamide, **R**enal tubular acidosis, **D**iarrhea, **U**reteroenteric fistula, **P**ancreatic fistula. * **Loop/Thiazide Diuretics** cause metabolic alkalosis, hypokalemia, and hyperuricemia.

Question 123: All causes of metabolic acidosis, except:

A. Methanol
B. Ethanol
C. Salicylate
D. Isopropanol (Correct Answer)

Explanation: **Explanation:** The key to solving this question lies in understanding the metabolic byproducts of various alcohols. **Isopropanol (Isopropyl alcohol)** is metabolized by alcohol dehydrogenase into **acetone**. Unlike other alcohols, acetone is a ketone but not a ketoacid; it does not dissociate to release hydrogen ions. Therefore, isopropanol poisoning typically presents with **ketonemia and ketonuria without metabolic acidosis**. The presence of an "osmolal gap" without a "metabolic acidosis" is a classic diagnostic hallmark of isopropanol ingestion. **Analysis of Incorrect Options:** * **Methanol:** Metabolized to **formic acid**, which causes a profound High Anion Gap Metabolic Acidosis (HAGMA) and retinal toxicity. * **Ethanol:** Can lead to metabolic acidosis through the accumulation of **lactate** (due to an increased NADH/NAD+ ratio) and the formation of **ketoacids** (Ethanol-induced Ketoacidosis). * **Salicylate:** Causes a complex acid-base disturbance. While it initially triggers respiratory alkalosis, it eventually causes **HAGMA** by interfering with the Krebs cycle and uncoupling oxidative phosphorylation, leading to the accumulation of organic acids. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** "MUDPILES" (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Isopropanol Clue:** If a patient smells of "fruity" acetone but has a **normal pH and normal anion gap**, suspect Isopropanol. * **Ethylene Glycol:** Look for HAGMA plus **calcium oxalate crystals** (envelope-shaped) in urine and acute renal failure.

Question 124: In a solution, the concentration of hydrogen ion is 1 x 10^-7 moles/litre at 25°C. What will be the pH of the solution?

A. Three
B. Seven (Correct Answer)
C. Nine
D. Twelve

Explanation: **Explanation:** The pH of a solution is defined as the negative logarithm (to the base 10) of the hydrogen ion concentration $[H^+]$. This concept is fundamental in biochemistry for understanding acid-base homeostasis. **The Calculation:** The formula used is: **pH = -log₁₀ [H⁺]** Given: $[H⁺] = 1 \times 10^{-7}$ moles/litre pH = -log₁₀ $(10^{-7})$ pH = -(-7) log₁₀ (10) **pH = 7** At 25°C, a pH of 7 represents a **neutral solution**, where the concentration of hydrogen ions $[H^+]$ exactly equals the concentration of hydroxyl ions $[OH^-]$. **Analysis of Incorrect Options:** * **Option A (Three):** A pH of 3 corresponds to $[H^+] = 10^{-3}$ mol/L. This is 10,000 times more acidic than the given solution. * **Option C (Nine):** A pH of 9 corresponds to $[H^+] = 10^{-9}$ mol/L. This represents an alkaline solution. * **Option D (Twelve):** A pH of 12 corresponds to $[H^+] = 10^{-12}$ mol/L. This represents a strongly basic solution. **High-Yield Clinical Pearls for NEET-PG:** * **Henderson-Hasselbalch Equation:** $pH = pKa + \log ([Salt]/[Acid])$. This is the gold standard for calculating the pH of buffer systems like the bicarbonate-carbonic acid buffer in blood. * **Normal Blood pH:** The physiological pH of arterial blood is tightly regulated between **7.35 and 7.45**. * **Temperature Dependency:** The neutrality point (pH 7) is specific to 25°C. As temperature increases, the $Kw$ (ionic product of water) changes, though the solution remains neutral. * **Logarithmic Scale:** Remember that a change of 1 pH unit represents a **10-fold change** in $[H^+]$ concentration.

Question 125: Calculate the anion gap from the following information: Na+ = 137 mmol/L, K+ = 4 mmol/L, Cl- = 110 mmol/L, HCO3- = 15 mmol/L.

A. 22 mmol/L
B. 16 mmol/L
C. 10 mmol/L
D. 12 mmol/L (Correct Answer)

Explanation: ### Explanation **1. Understanding the Correct Answer (Option D: 12 mmol/L)** The Serum Anion Gap (SAG) represents the difference between measured cations and measured anions in the serum. It reflects the concentration of unmeasured anions (such as phosphates, sulfates, organic acids, and albumin). The standard formula used in clinical practice is: **Anion Gap = [Na⁺] – ([Cl⁻] + [HCO₃⁻])** *Note: While K⁺ is a cation, it is usually omitted from the formula because its concentration is low and relatively constant.* **Calculation:** * Anion Gap = 137 – (110 + 15) * Anion Gap = 137 – 125 = **12 mmol/L** A value of 12 mmol/L falls within the normal reference range (typically **8–12 mmol/L** or **10–14 mmol/L** depending on the lab). **2. Analysis of Incorrect Options** * **Option A (22 mmol/L):** This represents a **High Anion Gap Metabolic Acidosis (HAGMA)**. This occurs when acid anions (like lactate or ketones) accumulate. * **Option B (16 mmol/L):** This is slightly elevated. If K⁺ were included in the formula ([Na⁺ + K⁺] – [Cl⁻ + HCO₃⁻]), the result would be 16 mmol/L. However, the standard clinical formula excludes K⁺. * **Option C (10 mmol/L):** While this is within the normal range, it is mathematically incorrect based on the provided values. **3. NEET-PG High-Yield Pearls** * **Albumin Correction:** Albumin is the major unmeasured anion. For every **1 g/dL decrease** in serum albumin below normal (4 g/dL), the "normal" anion gap decreases by approximately **2.5 mmol/L**. * **MUDPILES:** The classic mnemonic for HAGMA causes (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactate, Ethylene glycol, Salicylates). * **Normal Anion Gap Metabolic Acidosis (NAGMA):** Also called hyperchloremic acidosis; common causes include Diarrhea and Renal Tubular Acidosis (RTA).

Question 126: Which of the following is NOT a cause of hypomagnesemia?

A. Gitelman syndrome
B. Re-feeding syndrome
C. Hyperaldosteronism
D. Rhabdomyolysis (Correct Answer)

Explanation: **Explanation:** The correct answer is **Rhabdomyolysis**, which is a cause of **hypermagnesemia**, not hypomagnesemia. **1. Why Rhabdomyolysis is the correct answer:** Magnesium is the second most abundant intracellular cation. In rhabdomyolysis, extensive muscle cell breakdown (lysis) causes the release of intracellular contents into the extracellular fluid. This leads to a triad of electrolyte elevations: **Hyperkalemia, Hyperphosphatemia, and Hypermagnesemia**. Additionally, associated acute kidney injury (AKI) further impairs the excretion of magnesium, worsening the elevation. **2. Analysis of Incorrect Options (Causes of Hypomagnesemia):** * **Gitelman Syndrome:** A salt-losing tubulopathy affecting the thiazide-sensitive NaCl cotransporter in the distal convoluted tubule. It characteristically presents with hypokalemia, metabolic alkalosis, and profound **hypomagnesemia** due to renal magnesium wasting. * **Re-feeding Syndrome:** Occurs when malnourished patients receive rapid nutritional replacement. Insulin release shifts magnesium, potassium, and phosphate from the blood **into the cells**, leading to life-threatening hypomagnesemia. * **Hyperaldosteronism:** Excess aldosterone causes expansion of extracellular fluid volume. This inhibits proximal tubular reabsorption of sodium and, by extension, magnesium, leading to increased urinary magnesium excretion. **Clinical Pearls for NEET-PG:** * **"The Magnesium-Potassium Link":** Refractory hypokalemia cannot be corrected until the underlying hypomagnesemia is treated, as magnesium is required to inhibit the ROMK channels in the kidney. * **Hypocalcemia Connection:** Severe hypomagnesemia causes functional hypoparathyroidism (inhibits PTH release and action), leading to hypocalcemia. * **ECG Findings:** Hypomagnesemia can lead to Torsades de Pointes; Hypermagnesemia causes bradycardia and heart block.

Question 127: Increased serum calcium is seen in all of the following conditions except?

A. Myxedema (Correct Answer)
B. Multiple myeloma
C. Primary hyperparathyroidism
D. Hyperthyroidism

Explanation: **Explanation:** The correct answer is **Myxedema (Hypothyroidism)**. In this condition, serum calcium levels are typically **normal or low**, but never increased. **1. Why Myxedema is the correct answer:** Myxedema refers to severe hypothyroidism. Thyroid hormones normally stimulate bone resorption (turnover). In hypothyroidism, there is a decrease in bone remodeling and a reduction in the release of calcium from the bone into the blood. Furthermore, some patients may have associated Vitamin D deficiency, leading to hypocalcemia rather than hypercalcemia. **2. Analysis of Incorrect Options (Conditions causing Hypercalcemia):** * **Multiple Myeloma:** This is a plasma cell dyscrasia where malignant cells produce "Osteoclast Activating Factors" (like IL-6 and RANKL). These factors stimulate osteoclasts to break down bone, leading to significant hypercalcemia and "punched-out" lytic lesions. * **Primary Hyperparathyroidism:** This is the most common cause of hypercalcemia in outpatient settings. Excess Parathyroid Hormone (PTH) increases bone resorption, enhances renal calcium reabsorption, and increases intestinal calcium absorption via Vitamin D activation. * **Hyperthyroidism:** Excess T3 and T4 have a direct stimulatory effect on osteoclasts, leading to increased bone turnover. Approximately 15-20% of thyrotoxic patients exhibit mild hypercalcemia. **NEET-PG High-Yield Pearls:** * **Most common cause of hypercalcemia (Outpatient):** Primary Hyperparathyroidism. * **Most common cause of hypercalcemia (Inpatient/Hospitalized):** Malignancy. * **Milk-Alkali Syndrome:** A classic triad of hypercalcemia, metabolic alkalosis, and renal failure due to excessive ingestion of calcium carbonate. * **ECG in Hypercalcemia:** Characterized by a **shortened QT interval**.

Question 128: Which buffer system has the highest pKa?

A. Phosphate
B. Bicarbonate
C. Intracellular proteins
D. Ammonia (Correct Answer)

Explanation: **Explanation** The pKa of a buffer system represents the pH at which the acid and its conjugate base are present in equal concentrations. A higher pKa indicates a weaker acid that holds onto its protons more tightly in alkaline environments. **Why Ammonia is Correct:** The **Ammonia ($NH_3/NH_4^+$) buffer system** has a **pKa of approximately 9.2–9.3**. This is significantly higher than the physiological pH of blood (7.4). In the distal tubule of the kidney, ammonia ($NH_3$) diffuses into the lumen and reacts with secreted $H^+$ ions to form ammonium ($NH_4^+$). Because the pKa is so high, the reaction almost exclusively favors the formation of $NH_4^+$, which is "trapped" in the urine (ion trapping). This makes it the most important system for the excretion of metabolic acids and the regeneration of bicarbonate. **Why the others are incorrect:** * **Phosphate ($HPO_4^{2-}/H_2PO_4^-$):** Has a **pKa of 6.8**. While it is an effective urinary and intracellular buffer because its pKa is close to physiological pH, it is lower than that of ammonia. * **Bicarbonate ($H_2CO_3/HCO_3^-$):** Has a **pKa of 6.1**. Despite having a pKa further from 7.4, it is the most important extracellular buffer because it is an open system (CO₂ regulated by lungs, $HCO_3^-$ by kidneys). * **Intracellular Proteins:** The most important protein buffer is Hemoglobin. The imidazole group of Histidine residues provides a **pKa of approximately 6.0–7.0**. **High-Yield NEET-PG Pearls:** * **Most important ECF buffer:** Bicarbonate system. * **Most important ICF buffer:** Proteins and Phosphate. * **Most important Urinary buffer:** Phosphate (for titratable acidity) and Ammonia (for non-titratable acidity). * **Maximum buffering capacity** occurs when **pH = pKa**.

Question 129: Widened anionic gap is not seen in which of the following conditions?

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

Explanation: ### Explanation The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. It represents unmeasured anions in the plasma (like phosphates, sulfates, and organic acids). Metabolic acidosis is classified into two types based on this gap: **High Anion Gap Metabolic Acidosis (HAGMA)** and **Normal Anion Gap Metabolic Acidosis (NAGMA)**. **Why Diarrhea is the correct answer:** Diarrhea is a classic cause of **NAGMA** (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 $HCO_3^-$ is balanced by an equal increase in $Cl^-$, the Anion Gap remains within the normal range (8–12 mEq/L). **Analysis of Incorrect Options (Causes of HAGMA):** * **Acute Renal Failure:** Failure to excrete fixed acids (phosphates and sulfates) leads to an accumulation of unmeasured anions, widening the gap. * **Lactic Acidosis:** Excess production of lactate (an unmeasured anion) during tissue hypoxia increases the gap. * **Diabetic Ketoacidosis:** The accumulation of ketone bodies (acetoacetate and beta-hydroxybutyrate) adds unmeasured anions to the blood, widening the gap. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA:** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Key Distinction:** If the question mentions **Hyperchloremia**, always think of NAGMA (e.g., Diarrhea or RTA).

Question 130: A 27-year-old woman presents to the emergency room with a panic attack. She appears healthy except for tachycardia and a respiratory rate of 30. Electrolytes include calcium 10.0 mg/dL, albumin 4.0 g/dL, phosphorus 0.8 mg/dL, and magnesium 1.5 mEq/L. Arterial blood gases include pH of 7.56, PCO2 21 mm Hg, and PO2 99 mm Hg. Which of the following is the most likely cause of the hypophosphatemia?

A. Hypomagnesemia
B. Hyperparathyroidism
C. Respiratory alkalosis with intracellular shift (Correct Answer)
D. Poor dietary intake

Explanation: ### Explanation **Correct Option: C. Respiratory alkalosis with intracellular shift** The patient is presenting with a **panic attack** leading to hyperventilation, evidenced by a respiratory rate of 30 and ABG findings of **respiratory alkalosis** (pH 7.56, $PCO_2$ 21 mmHg). **Pathophysiology:** In respiratory alkalosis, the decrease in $PCO_2$ causes an increase in intracellular pH. This stimulates the enzyme **phosphofructokinase**, the rate-limiting step of glycolysis. Increased glycolytic activity consumes inorganic phosphate to produce phosphorylated glucose intermediates (like ATP and 2,3-BPG). This creates a gradient that drives phosphate from the extracellular fluid into the cells. This "intracellular shift" is the most common cause of profound hypophosphatemia in hospitalized patients with acute respiratory alkalosis. --- ### Why other options are incorrect: * **A. Hypomagnesemia:** While low magnesium can coexist with electrolyte imbalances, it does not directly cause an acute, severe drop in phosphate in the setting of hyperventilation. * **B. Hyperparathyroidism:** Primary hyperparathyroidism causes hypophosphatemia via renal wasting (phosphaturia), but it is typically associated with **hypercalcemia**. This patient’s calcium (10.0 mg/dL) is normal. * **D. Poor dietary intake:** Isolated dietary deficiency is a rare cause of severe hypophosphatemia because the kidneys can efficiently compensate by increasing phosphate reabsorption. It would not explain the acute presentation. --- ### NEET-PG High-Yield Pearls: * **The "Shift" Rule:** Just as insulin and alkalosis cause potassium to shift intracellularly, **respiratory alkalosis** is a classic trigger for an intracellular **phosphate shift**. * **Clinical Presentation:** Severe hypophosphatemia (<1.0 mg/dL) can lead to **rhabdomyolysis**, hemolysis, and respiratory muscle weakness due to ATP depletion. * **Panic Attack Triad:** Hyperventilation $\rightarrow$ Respiratory Alkalosis $\rightarrow$ Hypophosphatemia + Hypocalcemia symptoms (due to increased calcium binding to albumin).

Practice by Chapter

Acid-Base Chemistry and Buffers

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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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