**Core Concept**
Furosemide, a loop diuretic, works by inhibiting the sodium-potassium-chloride cotransporter (NKCC2) in the ascending limb of the loop of Henle, leading to increased urine production and natriuresis. This results in a loss of potassium, calcium, and magnesium ions, causing various metabolic disturbances.

**Why the Correct Answer is Right**
The chronic use of furosemide in this patient can lead to hypokalemia due to excessive potassium excretion in the urine. This occurs because the inhibition of NKCC2 in the loop of Henle disrupts the balance of potassium ions in the body, leading to a decrease in serum potassium levels. As a result, the patient is at risk of developing hypokalemic cardiac arrhythmias, muscle weakness, and other complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypercalcemia is not a typical side effect of furosemide. While furosemide can cause a mild increase in serum calcium levels, it is not a significant concern in this patient.

**Option B:** Hyperphosphatemia is not directly related to furosemide use. Furosemide can cause a mild increase in serum phosphate levels, but this is not a major concern.

**Option C:** Metabolic alkalosis is a possible side effect of furosemide, but it is not the most likely metabolic abnormality in this patient. Furosemide can cause a mild increase in serum bicarbonate levels, but this is not as significant as the risk of hypokalemia.

**Clinical Pearl / High-Yield Fact**
Furosemide-induced hypokalemia is a common side effect of loop diuretics, and patients with CHF are particularly susceptible due to their compromised renal function. Monitoring serum potassium levels and adjusting the furosemide dose accordingly is essential to prevent complications.

**Correct Answer: C. Metabolic alkalosis.**