Acid-Base and Electrolyte Balance — MCQs

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

Question 21: What is the concentration of Sodium (Na) in mEq/L in normal saline?

A. 77
B. 109
C. 130
D. 154 (Correct Answer)

Explanation: **Explanation:** Normal Saline (0.9% NaCl) is an isotonic crystalloid solution widely used in clinical practice. The concentration of 0.9% means there are **0.9 grams of Sodium Chloride in every 100 mL** of solution, which equates to **9 grams per Liter**. To calculate the mEq/L: 1. **Molecular Weight of NaCl:** ~58.5 g/mol. 2. **Calculation:** 9g / 58.5 = 0.1538 moles/L. 3. **Conversion:** 0.1538 moles = 154 millimoles (mmol). Since Sodium has a valence of 1, **154 mmol/L = 154 mEq/L**. Consequently, Normal Saline contains 154 mEq/L of Na⁺ and 154 mEq/L of Cl⁻, giving it a total osmolarity of **308 mOsm/L**. **Analysis of Incorrect Options:** * **A (77 mEq/L):** This is the sodium concentration of **Half-Normal Saline (0.45% NaCl)**. * **B (109 mEq/L):** This is the chloride concentration found in **Ringer’s Lactate**. * **C (130 mEq/L):** This is the sodium concentration found in **Ringer’s Lactate**, making it more physiological than NS. **Clinical Pearls for NEET-PG:** * **Hyperchloremic Metabolic Acidosis:** Large volumes of Normal Saline can cause this because its chloride content (154 mEq/L) is significantly higher than plasma chloride (98–107 mEq/L). * **Isotonicity:** While called "Normal," its osmolarity (308) is slightly higher than normal plasma osmolarity (285–295 mOsm/L). * **Drug of Choice:** NS is the preferred fluid for initial resuscitation in hypovolemic shock and the only fluid compatible with blood transfusions.

Question 22: Magnesium level in blood increases in which of the following conditions?

A. Uncontrolled Diabetes Mellitus
B. Liver cirrhosis
C. Kidney failure (Correct Answer)
D. Chronic alcoholism

Explanation: **Explanation:** **Correct Answer: C. Kidney failure** **Mechanism:** The kidneys are the primary organs responsible for magnesium homeostasis. Approximately 70% of serum magnesium is filtered at the glomerulus, and the majority is reabsorbed in the thick ascending limb of the Loop of Henle. In **Kidney Failure** (Acute Kidney Injury or Chronic Kidney Disease), the glomerular filtration rate (GFR) decreases significantly. This leads to a reduced excretory capacity, causing magnesium to accumulate in the blood (**Hypermagnesemia**). This is particularly exacerbated if the patient consumes magnesium-containing antacids or laxatives. **Analysis of Incorrect Options:** * **Uncontrolled Diabetes Mellitus:** Causes **hypomagnesemia**. Osmotic diuresis induced by glucosuria leads to excessive renal loss of magnesium. * **Liver Cirrhosis:** Often associated with low magnesium levels due to malnutrition, the use of diuretics (like Spironolactone or Furosemide), and altered renal handling. * **Chronic Alcoholism:** A classic cause of **hypomagnesemia**. Alcohol acts as a tubular toxin that inhibits magnesium reabsorption; additionally, alcoholics often suffer from poor dietary intake and chronic diarrhea. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Serum Magnesium:** 1.7 to 2.2 mg/dL. * **ECG Changes in Hypermagnesemia:** Similar to hyperkalemia (prolonged PR interval, widened QRS, and peaked T-waves). * **Clinical Sign:** Loss of deep tendon reflexes (DTRs) is an early sign of magnesium toxicity (usually seen at levels >4-5 mEq/L). * **Antidote:** **Calcium gluconate** is used to antagonize the cardiotoxic effects of severe hypermagnesemia.

Question 23: In a solution, the concentration of hydrogen ion (H+) is 1 x 10-6 moles/litre. What will be the pH of the solution?

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

Explanation: **Explanation:** The pH of a solution is a measure of its acidity or alkalinity, defined as the **negative logarithm (base 10) of the hydrogen ion concentration [H+]**. The mathematical formula is: **pH = -log₁₀ [H+]** Given that [H+] = 1 x 10⁻⁶ mol/L: * pH = -log₁₀ (10⁻⁶) * pH = - (-6) * **pH = 6** **Analysis of Options:** * **Option B (Correct):** As calculated above, a concentration of 10⁻⁶ corresponds to a pH of 6. This solution is slightly acidic (pH < 7). * **Option A (Incorrect):** A pH of 3 corresponds to [H+] of 10⁻³ mol/L, which is 1,000 times more acidic than the given solution. * **Option C (Incorrect):** A pH of 9 corresponds to [H+] of 10⁻⁹ mol/L, representing an alkaline solution. * **Option D (Incorrect):** A pH of 12 corresponds to [H+] of 10⁻¹² mol/L, representing a strongly basic solution. **Clinical Pearls for NEET-PG:** 1. **Logarithmic Scale:** Remember that pH is a logarithmic scale. A change of **1 pH unit** represents a **10-fold change** in H+ concentration. 2. **Normal Blood pH:** The physiological pH of arterial blood is tightly regulated between **7.35 and 7.45**. 3. **Henderson-Hasselbalch Equation:** For clinical acid-base disorders, remember: **pH = pKa + log ([HCO₃⁻] / 0.03 × PCO₂)**. This relates metabolic (bicarbonate) and respiratory (CO₂) components. 4. **Inverse Relationship:** As the concentration of H+ ions increases, the pH value decreases (Acidosis).

Question 24: What is the tonicity of a 10% dextrose solution?

A. Isotonic
B. Hypotonic
C. Hypertonic (Correct Answer)
D. None of the above

Explanation: **Explanation:** The tonicity of a solution is determined by its effective osmolality relative to plasma (normal range: 275–295 mOsm/kg). **Why Hypertonic is correct:** A 10% dextrose solution (D10W) contains 10 grams of glucose per 100 mL, which equates to 100 grams per liter. Since the molecular weight of glucose is 180, the osmolarity is calculated as: $(100 / 180) \times 1000 \approx 505 \text{ mOsm/L}$. Because 505 mOsm/L is significantly higher than the plasma osmolality (~290 mOsm/L), the solution is **hypertonic** at the time of administration. **Analysis of Incorrect Options:** * **Isotonic:** A 5% dextrose solution (D5W) is considered isotonic in the bag (~252–278 mOsm/L). 10% dextrose is double that concentration, making it hypertonic. * **Hypotonic:** Solutions with an osmolarity significantly lower than 275 mOsm/L (e.g., 0.45% Normal Saline) are hypotonic. D10W far exceeds this threshold. **Clinical Pearls for NEET-PG:** 1. **The "Physiological Paradox":** While D10W is **hypertonic in the bottle**, it becomes **hypotonic in the body**. Once infused, glucose is rapidly metabolized by insulin, leaving behind "free water." 2. **Indications:** D10W is primarily used in the management of severe hypoglycemia and as part of parenteral nutrition. 3. **Administration:** Because of its high tonicity, D10W can cause vein irritation (thrombophlebitis). While D10W can often be given peripherally, solutions with higher concentrations (D25, D50) ideally require a central line. 4. **High-Yield Values:** * 0.9% NaCl (Normal Saline): Isotonic (308 mOsm/L) * 5% Dextrose in 0.9% NaCl (D5NS): Hypertonic (560 mOsm/L) * Ringer's Lactate: Isotonic (273 mOsm/L)

Question 25: Which of the following is associated with an increased anion gap?

A. Myasthenia Gravis
B. Chronic obstructive pulmonary disease
C. Diabetic coma (Correct Answer)
D. Nasogastric suctioning

Explanation: **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. A normal gap (8–12 mEq/L) represents unmeasured anions like albumin and phosphates. An **Increased Anion Gap Metabolic Acidosis (HAGMA)** occurs when fixed acids (organic acids) are added to the blood, consuming bicarbonate. **Why Diabetic Coma is Correct:** In Diabetic Ketoacidosis (DKA), insulin deficiency leads to the overproduction of **ketoacids** (β-hydroxybutyrate and acetoacetate). These organic acids dissociate, releasing $H^+$ ions that are buffered by $HCO_3^-$, while the unmeasured ketoacid anions increase the anion gap. **Analysis of Incorrect Options:** * **Myasthenia Gravis & COPD:** These conditions lead to **Respiratory Acidosis** due to hypoventilation and $CO_2$ retention. The anion gap remains normal because the primary pathology is a gas exchange issue, not the accumulation of fixed metabolic acids. * **Nasogastric (NG) Suctioning:** This causes **Metabolic Alkalosis**. Loss of gastric $HCl$ leads to a loss of chloride and hydrogen ions, typically resulting in a "contraction alkalosis" with a normal or slightly altered anion gap, but never HAGMA. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Normal Anion Gap Metabolic Acidosis (NAGMA):** Characterized by hyperchloremia; common causes include **Diarrhea** and **Renal Tubular Acidosis (RTA)**. * **Goldman’s Formula:** Used to calculate the expected $pCO_2$ in metabolic acidosis to check for respiratory compensation (Winters' Formula).

Question 26: Increased anion gap is found in which of the following disorders except?

A. Diabetic ketoacidosis
B. Starvation ketosis
C. Renal tubular acidosis (Correct Answer)
D. Lactic acidosis

Explanation: The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. A normal anion gap is typically **8–12 mEq/L**. 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 Renal Tubular Acidosis (RTA) is the Correct Answer: **Renal Tubular Acidosis** is a classic cause of **NAGMA** (also known as hyperchloremic metabolic acidosis). In RTA, there is either a failure to reabsorb bicarbonate (Proximal/Type 2) or a failure to excrete hydrogen ions (Distal/Type 1). To maintain electroneutrality as bicarbonate is lost, the kidneys retain **Chloride ($Cl^-$)**. Since the increase in chloride offsets the decrease in bicarbonate, the calculated anion gap remains within the normal range. ### Why Other Options are Incorrect: * **Diabetic Ketoacidosis (DKA) & Starvation Ketosis:** Both involve the accumulation of unmeasured anions called **ketones** (acetoacetate and beta-hydroxybutyrate). These consume bicarbonate, increasing the anion gap. * **Lactic Acidosis:** This occurs due to tissue hypoxia or sepsis, leading to the accumulation of **lactate**. Lactate is an unmeasured anion that replaces bicarbonate, resulting in HAGMA. ### NEET-PG High-Yield Pearls: * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA (Normal Gap):** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, **RTA**, Pancreatic fistula). * **Gold Standard:** Diarrhea is the most common cause of NAGMA globally, but RTA is the most common "renal" cause tested in exams.

Question 27: Which acid-base disturbances are typically associated with sepsis?

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

Explanation: In sepsis, the most characteristic acid-base disturbance is a **mixed disorder** consisting of **Metabolic Acidosis and Respiratory Alkalosis**. ### Why Option B is Correct: 1. **Metabolic Acidosis (High Anion Gap):** Sepsis leads to tissue hypoperfusion and cellular hypoxia. This forces cells into anaerobic metabolism, resulting in the overproduction of **lactic acid** (Lactic Acidosis). 2. **Respiratory Alkalosis:** This occurs early in sepsis due to a centrally mediated increase in respiratory drive. Endotoxins, fever, and the systemic inflammatory response syndrome (SIRS) stimulate the medullary respiratory center, leading to hyperventilation and a decrease in $PaCO_2$. ### Why Other Options are Incorrect: * **Option A:** Respiratory acidosis (high $PaCO_2$) only occurs in the terminal stages of sepsis if the patient develops respiratory muscle fatigue or ARDS; it is not the "typical" initial presentation. * **Options C & D:** Metabolic alkalosis is rare in sepsis unless there is significant concomitant vomiting or nasogastric suctioning. Sepsis is fundamentally a state of acid accumulation, not base excess. ### High-Yield Clinical Pearls for NEET-PG: * **Mixed Acid-Base Disorders:** When you see a low $HCO_3^-$ (metabolic acidosis) and a $PaCO_2$ lower than expected by Winters’ formula, suspect a primary respiratory alkalosis. * **Salicylate Poisoning:** This is the other classic condition that presents with the same mixed pattern (Metabolic Acidosis + Respiratory Alkalosis). * **Lactate:** Serum lactate levels are a key prognostic marker in the "Surviving Sepsis" guidelines; levels >2 mmol/L despite fluid resuscitation define septic shock.

Question 28: Which of the following is NOT a cause of metabolic alkalosis?

A. Mineralocorticoid deficiency (Correct Answer)
B. Bartter's syndrome
C. Thiazide diuretic therapy
D. Recurrent vomiting

Explanation: **Explanation:** Metabolic alkalosis is characterized by an increase in plasma bicarbonate ($HCO_3^-$) and a rise in arterial pH. To understand the options, we must look at the role of the distal tubule and aldosterone. **Why Option A is Correct:** **Mineralocorticoid deficiency** (e.g., Addison’s disease) leads to **Metabolic Acidosis**, not alkalosis. Aldosterone normally acts on the principal cells to reabsorb $Na^+$ and secrete $K^+$, and on intercalated cells to secrete $H^+$. A deficiency results in the retention of $H^+$ ions and $K^+$ (hyperkalemia), leading to Normal Anion Gap Metabolic Acidosis (Type 4 RTA). **Why the other options are incorrect:** * **Bartter’s Syndrome:** A genetic defect in the thick ascending limb (NKCC2 transporter) that mimics loop diuretics. It causes salt wasting, activation of the RAAS, and increased distal delivery of $Na^+$, leading to increased $H^+$ secretion and **metabolic alkalosis**. * **Thiazide Diuretics:** These inhibit the $Na^+/Cl^-$ symporter in the distal tubule. The resulting volume contraction and increased distal $Na^+$ delivery stimulate aldosterone, promoting $K^+$ and $H^+$ loss, causing **contraction alkalosis**. * **Recurrent Vomiting:** Gastric juice is rich in $HCl$. Loss of $H^+$ ions directly increases plasma $HCO_3^-$. Additionally, the loss of chloride (hypochloremia) and volume depletion maintain the alkalosis by stimulating the kidneys to reabsorb bicarbonate. **High-Yield Clinical Pearls for NEET-PG:** * **Saline-Responsive Alkalosis:** Caused by vomiting or diuretics (Urinary $Cl^-$ < 10-20 mEq/L). * **Saline-Resistant Alkalosis:** Caused by Mineralocorticoid excess (e.g., Conn’s syndrome, Cushing’s) or genetic tubulopathies like Bartter’s/Gitelman’s (Urinary $Cl^-$ > 20 mEq/L). * **Rule of Thumb:** Mineralocorticoid **excess** causes alkalosis; **deficiency** causes acidosis.

Question 29: Magnesium deficiency is caused by which of the following conditions?

A. Prolonged artificial ventilation
B. Small bowel resection
C. Renal disease
D. Liver cirrhosis (Correct Answer)

Explanation: **Explanation:** **Correct Option: D (Liver Cirrhosis)** Magnesium deficiency (hypomagnesemia) is a common electrolyte abnormality in patients with liver cirrhosis, particularly those with alcoholic etiology. The underlying mechanisms include: 1. **Poor Dietary Intake:** Chronic alcoholics often have nutritional deficiencies. 2. **Increased Renal Loss:** Alcohol has a direct diuretic effect, inhibiting tubular reabsorption of magnesium. 3. **Secondary Hyperaldosteronism:** Cirrhosis leads to activation of the Renin-Angiotensin-Aldosterone System (RAAS). High aldosterone levels promote magnesium excretion in the distal renal tubules. 4. **Diuretic Therapy:** Use of loop or thiazide diuretics to manage ascites further depletes magnesium. **Analysis of Incorrect Options:** * **A. Prolonged artificial ventilation:** This is more commonly associated with respiratory alkalosis (due to CO2 washout) or ventilator-associated pneumonia, but it does not directly cause magnesium depletion. * **B. Small bowel resection:** While massive resection (Short Bowel Syndrome) can cause malabsorption, it is less frequently cited as a primary cause in standard NEET-PG clinical scenarios compared to the systemic metabolic impact of cirrhosis. * **C. Renal disease:** Chronic Kidney Disease (CKD) typically leads to **Hypermagnesemia** (high magnesium) because the kidneys are unable to excrete magnesium effectively. **High-Yield Facts for NEET-PG:** * **Refractory Hypokalemia:** If a patient’s potassium levels do not normalize with supplementation, always check Magnesium levels. Hypomagnesemia must be corrected first to fix hypokalemia. * **ECG Changes:** Similar to hypokalemia, including prolonged PR and QT intervals and T-wave flattening. It can lead to *Torsades de pointes*. * **Drug-induced Hypomagnesemia:** Common culprits include Amphotericin B, Cisplatin, Aminoglycosides, and Proton Pump Inhibitors (PPIs).

Question 30: Hypomagnesemia is associated with which of the following?

A. Alcoholism (Correct Answer)
B. Hypothyroidism
C. Both
D. None

Explanation: **Explanation:** **1. Why Alcoholism is Correct:** Chronic alcoholism is the most common cause of hypomagnesemia in clinical practice. The underlying mechanisms are multifactorial: * **Renal Loss:** Ethanol acts as a direct tubular toxin, inhibiting the reabsorption of magnesium in the loop of Henle and distal tubules, leading to hypermagnesuria. * **Nutritional Deficiency:** Alcoholics often have poor dietary intake of magnesium-rich foods. * **Gastrointestinal Loss:** Frequent diarrhea and malabsorption associated with chronic alcohol use further deplete magnesium stores. **2. Why Hypothyroidism is Incorrect:** Hypothyroidism is typically associated with **hypermagnesemia**, not hypomagnesemia. Thyroid hormones influence renal clearance; in hypothyroid states, there is a decrease in the renal excretion of magnesium, leading to elevated serum levels. Conversely, *hyperthyroidism* is a known cause of hypomagnesemia due to increased renal clearance and bone turnover. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Refractory Hypokalemia" Rule:** Hypomagnesemia often coexists with hypokalemia. If potassium levels do not normalize despite aggressive supplementation, check magnesium levels. Magnesium is a cofactor for the ROMK channels; its deficiency leads to excessive potassium secretion in the distal tubule. * **Hypocalcemia Connection:** Low magnesium inhibits the release of Parathyroid Hormone (PTH) and causes end-organ resistance to PTH, leading to hypocalcemia. * **ECG Findings:** Look for prolonged PR and QT intervals, and the classic "Torsades de Pointes" (treated with IV Magnesium Sulfate). * **Drug-Induced Causes:** Always remember PPIs (long-term use), Aminoglycosides, Amphotericin B, and Cisplatin as common triggers for hypomagnesemia.

Practice by Chapter

Acid-Base Chemistry and Buffers

Practice Questions

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

Practice Questions

Respiratory and Metabolic Alkalosis

Practice Questions

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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