Which patient is at MOST risk for developing Digoxin toxicity?

• A. A 30-year-old male patient with heart failure and hyperglycemia
• B. B 82-year-old male patient with atrial fibrillation and magnesium level of 1 mg/dL
• C. C 45-year-old female with potassium level of 4.2 mEq/L
• D. D 50-year-old female with a calcium level of 9 mg/dL

Answer: (B)

Digoxin has a very narrow therapeutic window, and its toxicity risk rises when electrolytes are imbalanced, especially magnesium. Low magnesium levels potentiate digoxin’s effect on the Na+/K+-ATPase pump, leading to more intracellular calcium, increased electrical instability, and a higher chance of dangerous arrhythmias. An older patient already has greater susceptibility due to age-related changes in clearance, but the critical factor here is hypomagnesemia (magnesium 1 mg/dL). The other scenarios show normal potassium and calcium levels (and a non-direct risk from hyperglycemia), so they’re less likely to tip into toxicity. Hence, the patient with the low magnesium level is most at risk.