Electrolyte Disorders USMLE Step 1 Practice Question

A 59-year-old man with chronic alcoholism presents with tremor, generalized weakness, and palpitations. Vital signs show HR 112/min, BP 148/92, RR 18, temp 37°C. ECG demonstrates polymorphic ventricular tachycardia with characteristic twisting of QRS complexes around the baseline. Laboratory studies reveal magnesium 0.8 mEq/L, potassium 3.0 mEq/L, and normal calcium. He denies recent diuretic use. Which electrolyte abnormality best explains his arrhythmia?