Acid-Base and Electrolyte Balance — MCQs

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

Question 111: Which electrolyte imbalance is most commonly associated with fatigue, myalgia, and weakness?

A. Hypokalemia (Correct Answer)
B. Hyponatremia
C. Hypomagnesemia
D. Hypermagnesemia

Explanation: **Explanation:** **Hypokalemia (Option A)** is the correct answer because potassium is the primary intracellular cation essential for maintaining the resting membrane potential of excitable tissues. When serum potassium levels drop, the cell membrane becomes hyperpolarized, making it more difficult to trigger an action potential. This manifests clinically as **skeletal muscle weakness, fatigue, and myalgias**. In severe cases, this can progress to paralysis (starting from the lower extremities) and respiratory failure. **Why the other options are incorrect:** * **Hyponatremia (Option B):** While it can cause malaise, its hallmark symptoms are primarily neurological due to cerebral edema (e.g., headache, confusion, seizures, and coma) rather than primary muscle weakness. * **Hypomagnesemia (Option C):** Though it often coexists with hypokalemia, its classic presentation is neuromuscular irritability (tetany, tremors, and hyperreflexia) rather than generalized weakness and myalgia. * **Hypermagnesemia (Option D):** This typically causes loss of deep tendon reflexes (DTRs), hypotension, and respiratory depression, but it is much less common than hypokalemia in clinical practice. **High-Yield Clinical Pearls for NEET-PG:** * **ECG Findings in Hypokalemia:** Look for flattened T-waves, prominent **U-waves**, and ST-segment depression. * **Muscle Complication:** Severe hypokalemia can lead to **rhabdomyolysis** due to impaired vasodilation in skeletal muscle during exercise. * **Refractory Hypokalemia:** If potassium levels do not normalize with supplementation, always check and correct **Magnesium** levels first, as magnesium is a cofactor for the ROMK channels in the kidney.

Question 112: Which of the following does not cause hyperkalemia?

A. Crush syndrome
B. Hemolysis
C. Renal failure
D. Intestinal obstruction (Correct Answer)

Explanation: **Explanation:** The correct answer is **Intestinal obstruction**. **1. Why Intestinal Obstruction causes Hypokalemia (not Hyperkalemia):** Intestinal obstruction leads to repeated vomiting and the sequestration of fluid into the bowel lumen ("third-spacing"). Gastric juice is rich in hydrogen and chloride, but also contains significant potassium. Loss of gastric contents leads to **metabolic alkalosis**. In alkalotic states, hydrogen ions move out of cells to compensate, causing potassium to move **into** the cells to maintain electroneutrality. Furthermore, volume depletion activates the Renin-Angiotensin-Aldosterone System (RAAS), which increases renal potassium excretion. Thus, intestinal obstruction typically results in **hypokalemia**. **2. Why the other options cause Hyperkalemia:** * **Crush Syndrome:** Massive muscle injury causes **rhabdomyolysis**. Since potassium is the primary intracellular cation, the destruction of muscle cells releases massive amounts of potassium into the extracellular fluid. * **Hemolysis:** Similar to muscle cells, Red Blood Cells (RBCs) contain high concentrations of potassium. Lysis of RBCs (whether in vivo or in vitro) releases this potassium into the plasma. * **Renal Failure:** The kidney is the primary organ responsible for potassium excretion (via the distal convoluted tubule and collecting duct). In renal failure, a decreased Glomerular Filtration Rate (GFR) and tubular dysfunction lead to potassium retention. **High-Yield Clinical Pearls for NEET-PG:** * **ECG in Hyperkalemia:** Tall "tented" T-waves, PR interval prolongation, and widening of the QRS complex (forming a sine wave pattern in severe cases). * **Pseudohyperkalemia:** Often caused by hemolysis during blood collection or prolonged application of a tourniquet. * **Insulin & Beta-2 Agonists:** Both shift potassium **into** cells and are used in the emergency management of hyperkalemia.

Question 113: In metabolic alkalosis, which of the following occurs?

A. Gain in fixed acid
B. Loss of base
C. Hyperkalemia
D. Rise in base excess (Correct Answer)

Explanation: ### Explanation **1. Why "Rise in Base Excess" is Correct:** Base Excess (BE) is a measure of the metabolic component of an acid-base disturbance. It represents the amount of strong acid or base required to return the blood pH to 7.40 at a $PCO_2$ of 40 mmHg. * In **metabolic alkalosis**, there is an accumulation of bicarbonate ($HCO_3^-$) or a loss of hydrogen ions ($H^+$). * This results in a **positive Base Excess** (typically > +2 mEq/L). Therefore, a "rise" in base excess is a hallmark diagnostic feature of metabolic alkalosis. **2. Why the Other Options are Incorrect:** * **A. Gain in fixed acid:** This occurs in **metabolic acidosis** (e.g., lactic acidosis or ketoacidosis), which lowers the pH and decreases bicarbonate levels. * **B. Loss of base:** This is the primary mechanism of **metabolic acidosis** (e.g., severe diarrhea where bicarbonate is lost), leading to a negative base excess (base deficit). * **C. Hyperkalemia:** Metabolic alkalosis is typically associated with **Hypokalemia**. As $H^+$ ions move out of cells to compensate for the alkalosis, $K^+$ ions move into the cells to maintain electroneutrality. Additionally, in states like hyperaldosteronism, $H^+$ and $K^+$ are both lost in the urine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Compensation:** The body compensates for metabolic alkalosis via **respiratory hypoventilation** (increasing $PCO_2$), though this is limited by the hypoxic drive. * **Chloride Responsiveness:** Metabolic alkalosis is classified into **Saline-responsive** (Urinary $Cl^-$ < 10 mEq/L, e.g., vomiting) and **Saline-resistant** (Urinary $Cl^-$ > 20 mEq/L, e.g., Conn’s syndrome). * **Paradoxical Aciduria:** In cases of volume depletion + hypokalemia + alkalosis, the kidney reabsorbs $Na^+$ in exchange for $H^+$ (instead of $K^+$), leading to acidic urine despite systemic alkalosis.

Question 114: Prominent U waves on the ECG is a feature of which of the following conditions?

A. Hyponatremia
B. Hypernatremia
C. Hypokalemia (Correct Answer)
D. Hyperkalemia

Explanation: **Explanation:** The correct answer is **Hypokalemia (Option C)**. **1. Why Hypokalemia is Correct:** Potassium is the primary intracellular cation responsible for the repolarization phase of the cardiac action potential. In hypokalemia (serum $K^+ < 3.5$ mEq/L), the resting membrane potential becomes more negative, and the duration of the action potential is prolonged. This delay in ventricular repolarization manifests on the ECG as: * **Prominent U waves:** A positive wave following the T wave (most characteristic). * **Flattening or inversion of T waves.** * **ST-segment depression.** * **Prolonged PR interval.** **2. Why the Other Options are Incorrect:** * **Hyperkalemia (Option D):** Characterized by "Tall Peaked T waves" (earliest sign), followed by loss of P waves, QRS widening, and eventually a "Sine wave" pattern leading to asystole. * **Hyponatremia and Hypernatremia (Options A & B):** Sodium imbalances primarily affect neurological status (cerebral edema or shrinkage) and volume status. They do not typically produce specific, diagnostic ECG changes like potassium imbalances do. **3. High-Yield Clinical Pearls for NEET-PG:** * **U wave origin:** Likely represents delayed repolarization of the Purkinje fibers or Mid-myocardial (M) cells. * **Hypomagnesemia:** Often co-exists with hypokalemia and can also cause U waves and Torsades de Pointes. * **Mnemonic for Hypokalemia ECG:** "6 **L**'s" — **L**ow T wave, **L**ine depressed (ST), **L**ong PR, **L**ethal ventricular arrhythmias, **L**arge U wave, **L**ax muscles (weakness). * **Digitalis toxicity:** Can also cause U waves, but usually alongside a "scooped" ST segment.

Question 115: A buffer that is most effective at a pH of about 4.5 is?

A. Acetate buffer (Correct Answer)
B. Bicarbonate buffer
C. Phosphate buffer
D. Tris buffer

Explanation: ### Explanation The effectiveness of a buffer is determined by its **pKa value**. According to the Henderson-Hasselbalch equation, a buffer is most efficient at resisting pH changes when the pH of the solution is equal to its pKa (pH = pKa). Generally, the effective buffering range is **pH = pKa ± 1**. **1. Why Acetate Buffer is Correct:** The pKa of acetic acid (acetate buffer) is approximately **4.76**. Since 4.5 falls within the optimal range (3.76 to 5.76) and is very close to the pKa, acetate is the most effective buffer among the choices for a pH of 4.5. **2. Analysis of Incorrect Options:** * **Bicarbonate Buffer (pKa ≈ 6.1):** This is the most important extracellular buffer in the human body. Its effective range is roughly 5.1 to 7.1. At a pH of 4.5, it would be almost entirely protonated and ineffective. * **Phosphate Buffer (pKa ≈ 6.8):** This is a major intracellular and urinary buffer. Its effective range is 5.8 to 7.8, making it unsuitable for a pH of 4.5. * **Tris Buffer (pKa ≈ 8.1):** Commonly used in laboratories for physiological studies, its effective range is 7.1 to 9.1. **3. High-Yield Clinical Pearls for NEET-PG:** * **Maximum Buffering Capacity:** Occurs when [Acid] = [Conjugate Base]. * **Blood pH:** Maintained at 7.4. The bicarbonate system is the primary ECF buffer because it is an "open system" (CO₂ can be exhaled by lungs). * **Intracellular Buffers:** Proteins (like Hemoglobin via imidazole groups of Histidine) and Phosphate are the primary buffers inside cells. * **Isoelectric Point (pI):** The pH at which a molecule carries no net electrical charge; not to be confused with pKa.

Question 116: All of the following are important causes of hyponatremia, EXCEPT:

A. Gastric fistula
B. Excessive vomiting
C. Excessive sweating (Correct Answer)
D. Hypothyroidism

Explanation: **Explanation:** The core concept in this question is the **relative loss of water versus sodium**. **Why "Excessive Sweating" is the correct answer:** Sweat is a **hypotonic** fluid, meaning it contains significantly more water than sodium (Sodium concentration in sweat is ~20–60 mEq/L, compared to ~140 mEq/L in plasma). When a person sweats excessively, they lose more water than salt, which initially leads to **hypernatremia** (increased serum sodium concentration). While subsequent thirst and water intake can sometimes lead to dilutional hyponatremia, in the context of pure fluid loss, sweating is classically associated with hypernatremic dehydration. **Analysis of Incorrect Options:** * **Gastric Fistula & Excessive Vomiting:** Gastric secretions contain significant amounts of sodium and chloride. Loss of these fluids leads to **isotonic or hypotonic volume depletion**. More importantly, the resulting volume depletion triggers the release of **ADH (Antidiuretic Hormone)**, which causes the kidneys to retain pure water, leading to dilutional **hyponatremia**. * **Hypothyroidism:** Severe hypothyroidism (Myxedema) causes hyponatremia primarily through non-osmotic release of ADH and a decrease in the glomerular filtration rate (GFR), which impairs the kidney's ability to excrete free water. **High-Yield Clinical Pearls for NEET-PG:** * **Pseudohyponatremia:** Occurs in states of extreme hyperlipidemia or hyperproteinemia (the aqueous phase of plasma is reduced, but sodium concentration in that phase is normal). * **SIADH:** A common cause of *euvolemic* hyponatremia. * **Rule of Thumb:** Most GI losses (vomiting, diarrhea, fistulas) lead to hyponatremia due to secondary ADH activation ("Volume takes precedence over Osmolality").

Question 117: Increased anion gap acidosis is caused by all of the following except:

A. Diuretics (Correct Answer)
B. Uremia
C. Ketoacidosis
D. Ethylene glycol

Explanation: **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. A High Anion Gap Metabolic Acidosis (HAGMA) occurs when fixed acids (unmeasured anions) are added to the blood, consuming bicarbonate without a corresponding increase in chloride. **Why Diuretics is the correct answer:** Diuretics (specifically loop and thiazide diuretics) typically cause **Metabolic Alkalosis**, not acidosis. They lead to the loss of $H^+$ and $K^+$ ions in the urine and cause "contraction alkalosis." Even in cases where diuretics cause acidosis (such as Acetazolamide), it is a **Normal Anion Gap Metabolic Acidosis (NAGMA)** due to direct bicarbonate loss, which is compensated by an increase in chloride (hyperchloremic acidosis). **Analysis of Incorrect Options (Causes of HAGMA):** * **Uremia:** In advanced renal failure, the kidneys fail to excrete organic acids (phosphates, sulfates), which accumulate as unmeasured anions. * **Ketoacidosis:** (Diabetic, alcoholic, or starvation) Results in the accumulation of acetoacetate and $\beta$-hydroxybutyrate. * **Ethylene glycol:** Metabolism of this antifreeze agent produces glycolic and oxalic acids, significantly raising the anion 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 (Hyperchloremic):** **HARDUP** (Hyperalimentation, Acetazolamide, Renal tubular acidosis, Diarrhea, Uretero-sigmoidostomy, Pancreatic fistula). * **Normal Anion Gap:** 8–12 mEq/L. * **Key Distinction:** If the question mentions "Hyperchloremia," always think of NAGMA. If it mentions "Unmeasured anions," think of HAGMA.

Question 118: For the medical condition of hepatic cirrhosis complicated by acute renal failure, select the associated acid-base disturbances.

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: ### Explanation The correct answer is **Metabolic acidosis and respiratory alkalosis**. This mixed acid-base disorder occurs due to the simultaneous presence of two distinct pathological processes: 1. **Metabolic Acidosis (due to Acute Renal Failure):** In acute renal failure (ARF), the kidneys fail to excrete fixed acids (like phosphates and sulfates) and cannot effectively regenerate bicarbonate. This leads to a **High Anion Gap Metabolic Acidosis (HAGMA)**. Additionally, if the cirrhosis leads to hepatorenal syndrome, decreased perfusion further worsens the metabolic acidosis. 2. **Respiratory Alkalosis (due to Hepatic Cirrhosis):** Patients with chronic liver disease/cirrhosis characteristically exhibit chronic hyperventilation. This is driven by increased levels of progesterone, ammonia, and cytokines which directly stimulate the **medullary respiratory center**, leading to a primary decrease in $PCO_2$. #### Analysis of Incorrect Options: * **Option A & C (Respiratory Acidosis):** Respiratory acidosis involves $CO_2$ retention (hypoventilation). This is rare in cirrhosis unless there is a secondary complication like hepatic encephalopathy leading to coma or severe pulmonary edema. * **Option D (Metabolic Alkalosis):** While cirrhotic patients on diuretics (like Furosemide) may develop metabolic alkalosis, the presence of **Acute Renal Failure** shifts the metabolic component toward acidosis due to the accumulation of organic acids. #### NEET-PG High-Yield Pearls: * **Mixed Acid-Base Disorders:** Always look for two separate organs failing. Here, Kidney = Metabolic; Liver = Respiratory. * **Salicylate Poisoning:** Another classic high-yield cause of mixed **Metabolic Acidosis and Respiratory Alkalosis**. * **Cirrhosis Hallmark:** Primary respiratory alkalosis is the most common acid-base finding in stable cirrhotic patients. * **Anion Gap:** In ARF, the Anion Gap is typically elevated ($>12 \text{ mEq/L}$).

Question 119: Increased serum calcium with decreased serum phosphate levels is typically seen in which of the following conditions?

A. Primary hyperparathyroidism (Correct Answer)
B. Secondary hyperparathyroidism due to Vitamin D deficiency
C. Malignancy with lytic bone lesions
D. Osteoporosis

Explanation: **Explanation:** The biochemical hallmark of **Primary Hyperparathyroidism (PHPT)** is the combination of **Hypercalcemia** and **Hypophosphatemia**. This occurs due to the autonomous overproduction of Parathyroid Hormone (PTH), usually from a parathyroid adenoma. * **Mechanism:** PTH increases serum calcium by stimulating osteoclastic bone resorption and increasing renal calcium reabsorption. Simultaneously, PTH acts on the proximal convoluted tubules of the kidney to **inhibit phosphate reabsorption** (phosphaturic effect), leading to decreased serum phosphate levels. **Analysis of Incorrect Options:** * **Secondary Hyperparathyroidism (Vitamin D deficiency):** Here, the primary issue is low calcium (due to poor absorption), which triggers a compensatory rise in PTH. While phosphate is low, **serum calcium is typically low or low-normal**, not increased. * **Malignancy with lytic bone lesions:** This causes hypercalcemia due to direct bone destruction. However, PTH is suppressed by the high calcium, and there is no PTH-mediated phosphaturia; therefore, **phosphate levels are usually normal or elevated.** * **Osteoporosis:** This is a quantitative reduction in bone mass where serum calcium, phosphate, and PTH levels typically remain **within the normal range.** **High-Yield Clinical Pearls for NEET-PG:** * **PTH Rule:** PTH "Pees" out Phosphate (Phosphaturic action). * **Urinary findings:** In PHPT, despite high serum calcium, there is often **Hypercalciuria** because the filtered load of calcium exceeds the kidney's reabsorptive capacity. * **Classic Triad:** "Stones (renal calculi), bones (osteitis fibrosa cystica), and abdominal groans (peptic ulcers/pancreatitis)." * **PTHrP:** In Humoral Hypercalcemia of Malignancy (e.g., Squamous cell CA of lung), PTHrP mimics PTH, also causing high Ca and low PO4, but **immunoreactive PTH will be low.**

Question 120: Which of the following conditions is characterized by raised calcium and phosphorous levels?

A. Chronic Renal Failure (CRF)
B. Vitamin D intoxication (Correct Answer)
C. Hyperparathyroidism
D. Pseudohypoparathyroidism

Explanation: **Explanation:** The correct answer is **Vitamin D intoxication**. To understand this, we must look at how Vitamin D regulates mineral homeostasis. **1. Why Vitamin D Intoxication is correct:** Vitamin D (specifically its active form, Calcitriol) acts on three main sites to increase plasma mineral levels: * **Intestines:** It significantly increases the absorption of both **Calcium and Phosphorus**. * **Kidneys:** It promotes the reabsorption of both minerals. * **Bone:** In high doses, it stimulates osteoclast activity, releasing both minerals into the blood. Therefore, toxicity leads to concurrent **Hypercalcemia and Hyperphosphatemia**. **2. Why the other options are incorrect:** * **Chronic Renal Failure (CRF):** Characterized by **Hypocalcemia** (due to decreased Calcitriol production) and **Hyperphosphatemia** (due to decreased renal excretion). * **Hyperparathyroidism:** PTH increases Calcium but decreases Phosphorus. PTH inhibits the proximal tubule phosphate transporter (NaPi-IIa), leading to phosphaturia. Thus, you see **Hypercalcemia and Hypophosphatemia**. * **Pseudohypoparathyroidism:** This is a state of PTH resistance. It mimics hypoparathyroidism, resulting in **Hypocalcemia and Hyperphosphatemia**. **3. NEET-PG High-Yield Pearls:** * **The "Rule of Opposites":** In most clinical scenarios (PTH-related or Renal-related), Calcium and Phosphorus move in opposite directions. Vitamin D is the primary exception where they **move in the same direction**. * **Sarcoidosis:** Patients with Sarcoidosis often present with the same biochemical profile (High Ca, High PO₄) because macrophages in granulomas contain 1-alpha-hydroxylase, which produces excess active Vitamin D. * **Metastatic Calcification:** High levels of both minerals (Calcium × Phosphorus product > 55) significantly increase the risk of calcium phosphate deposition in soft tissues.

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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