Sweat is an acceptable specimen for the measurement of which electrolyte?

Sweat is a useful specimen for the measurement of chloride, primarily because of its significance in diagnosing conditions like cystic fibrosis. In patients with cystic fibrosis, there is an abnormal transport of chloride ions across epithelial cells, leading to elevated levels of chloride in sweat. The sweat test measures this chloride concentration, providing critical diagnostic information.

In contrast, carbon dioxide is typically measured in blood or arterial blood gases; magnesium levels are generally assessed through serum samples, and potassium, while it can be found in sweat, is not typically used as a diagnostic marker in this fluid. Therefore, the use of sweat as a specimen for chloride measurement is both established and clinically relevant, making it the correct answer.