- The patient receiving furosemide therapy complains of muscle weakness and cramps, which is a common presentation of an electrolyte imbalance caused by this medication.

- Furosemide is a loop diuretic that acts on the ascending limb of the loop of Henle to inhibit the Na⁺-K⁺-2Cl⁻ symporter.
- This effect leads to increased excretion of sodium, chloride, and potassium in the urine. Among these, the loss of potassium is clinically significant, resulting in hypokalemia.

- Hypokalemia is the most common electrolyte abnormality seen with furosemide therapy.
- It manifests as muscle weakness, cramps, fatigue, and in severe cases, cardiac arrhythmias due to the essential role of potassium in muscle contraction and electrical conduction.

- While furosemide can influence other electrolytes such as sodium and calcium, hyponatremia and hypercalcemia are less commonly responsible for muscle weakness in this context.
- In fact, furosemide typically causes hypocalcemia rather than hypercalcemia due to increased calcium excretion.
- Hypermagnesemia is rare and usually associated with magnesium-containing medications or impaired renal excretion, not with furosemide use, which more commonly leads to hypomagnesemia.

Therefore, the most common electrolyte abnormality responsible for muscle weakness and cramps in a patient on furosemide therapy is hypokalemia.

Reference: Goodman & Gilman's: The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 35: Diuretics and Other Drugs Affecting Renal Function