A 32-year-old woman with a 5-year history of bulimia nervosa presents with generalized weakness, palpitations, and muscle cramps. Laboratory investigations reveal serum potassium 2.4 mEq/L and serum magnesium 1.3 mg/dL. Her 12-lead ECG shows the pattern marked **A** in the diagram: flattened T waves, prominent U waves (best seen in V2–V4), mild ST-segment depression, and apparent QT prolongation. She subsequently develops a brief run of polymorphic ventricular tachycardia. Which of the following is the MOST appropriate initial management step?

Question

Accepted Answer

C. Administer intravenous magnesium sulfate 1–2 g followed by oral potassium chloride 40–80 mEq/day. ## Why option B is correct

The ECG pattern marked **A** — flattened T waves, prominent U waves, and apparent QT prolongation (actually a fused QU interval) — is the hallmark of hypokalemia with concomitant hypomagnesemia. The clinical anchor is that **hypomagnesemia perpetuates renal potassium wasting and must be corrected FIRST before potassium repletion becomes effective**. Therefore, the most appropriate initial step is to administer intravenous magnesium sulfate 1–2 g (typically MgSO₄), followed by oral or intravenous potassium chloride. This dual approach addresses both the immediate ECG abnormality and the underlying electrolyte deficits that predispose to torsades de pointes.

## Why each distractor is wrong

- **Option A**: Although intravenous potassium is appropriate for severe hypokalemia with arrhythmias, administering K+ without first correcting hypomagnesemia is ineffective because magnesium depletion drives continued renal potassium wasting. This violates the cardinal rule: "Correct hypomagnesemia first."
- **Option C**: Intravenous calcium and insulin–dextrose are appropriate for hyperkalemia (peaked T waves, wide QRS, sine waves), not hypokalemia. The ECG pattern shown (**A**) is diagnostic of hypokalemia, not hyperkalemia.
- **Option D**: Emergency hemodialysis and pacemaker insertion are reserved for severe hyperkalemia with hemodynamic instability or refractory arrhythmias (corresponding to ECG patterns **C** and **D**), not hypokalemia. The pattern **A** is managed medically with electrolyte repletion.

**High-Yield:** In hypokalemia with hypomagnesemia, always replete magnesium first—potassium replacement fails until Mg²⁺ is restored, because hypomagnesemia perpetuates renal K⁺ wasting.

[cite: Harrison 21e Ch 313; UpToDate hypokalemia ECG]

Answer

A. Administer intravenous calcium gluconate 10 mL of 10% solution followed by insulin with dextrose

Answer

B. Administer intravenous potassium chloride 40 mEq/hour through a peripheral line with continuous cardiac monitoring

Answer

D. Initiate emergency hemodialysis and temporary pacemaker insertion

Explanation

Why option B is correct

Why each distractor is wrong

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