Which laboratory finding is most concerning for a child on furosemide who may be at risk for digoxin toxicity?

In children who are receiving furosemide, a loop diuretic, monitoring electrolyte levels is crucial due to the medication's potential side effects. Furosemide is known to cause the loss of important electrolytes, particularly potassium. Therefore, hypokalemia, or low potassium levels, is especially concerning because it can increase the risk of digoxin toxicity.

Digoxin, a medication used to treat various heart conditions, has a narrow therapeutic window, meaning the difference between an effective dose and a toxic dose is small. Potassium and digoxin interact in the body; low potassium levels can enhance digoxin's effects on the heart, potentially leading to toxicity. Symptoms of digoxin toxicity can include nausea, vomiting, gastrointestinal discomfort, and potentially life-threatening arrhythmias.

While other laboratory abnormalities such as hypomagnesemia, hypercalcemia, or hyperphosphatemia may have their respective clinical implications, they do not have the same direct relationship with digoxin toxicity as hypokalemia. Thus, hypokalemia is the most concerning finding for a child on furosemide at risk for digoxin toxicity.