Which condition is most likely to produce elevated plasma potassium?

Elevated plasma potassium, also known as hyperkalemia, can occur in various clinical situations, but certain conditions are more directly associated with it. Digitalis overdose is a significant cause of elevated potassium levels due to its effects on the sodium-potassium ATPase enzyme. When digitalis is taken in toxic amounts, it inhibits this enzyme, resulting in a decreased ability for cells to actively transport potassium into cells and sodium out of cells. As a consequence, potassium accumulates in the extracellular fluid leading to elevated plasma potassium levels.

In contrast, hypoparathyroidism primarily affects calcium levels rather than potassium. It can lead to decreased calcium due to insufficient parathyroid hormone, but does not directly influence potassium homeostasis. Cushing's syndrome, characterized by excess cortisol, typically leads to increased sodium retention and may cause hypokalemia rather than hyperkalemia due to the mineralocorticoid effect of cortisol. Diarrhea can lead to loss of potassium through the gastrointestinal tract, also resulting in hypokalemia rather than the elevation sought in this question.

Thus, the most likely condition to produce elevated plasma potassium is digitalis overdose, due to its direct effect on cellular potassium regulation.