A nurse in an urgent care clinic is caring for a client who reports muscle weakness after taking several over-the-counter diet pills. Which of the following laboratory results should be the nurse's priority?

Rationale: A. Calcium of 8.2 mg/dL is slightly low but less immediately life-threatening. B. Phosphorus 5.1 mEq/L is mildly elevated but usually less critical. C. Potassium 2.4 mEq/L is significantly low (normal 3.5–5.0 mEq/L) and can cause muscle weakness, cardiac dysrhythmias, and is a medical emergency. D. Sodium 145 mEq/L is within normal limits.