Which of the following may cause an increase in serum potassium levels?

Addison's Disease is associated with adrenal insufficiency, where the adrenal glands do not produce sufficient amounts of hormones, particularly cortisol and aldosterone. Aldosterone plays a critical role in regulating electrolyte balance, including potassium levels. When aldosterone levels are low, the kidney is less able to excrete potassium, leading to its accumulation in the blood and resulting in hyperkalemia (increased serum potassium levels).

In the context of this question, individuals with Addison's Disease often exhibit high serum potassium levels due to this impaired function of the adrenal glands, making this answer accurate.

In contrast, conditions like Cushing's Syndrome, which is characterized by excess cortisol production, can lead to increased aldosterone levels and, consequently, a greater excretion of potassium. Diarrhea and vomiting would typically lead to a loss of potassium, thereby decreasing serum potassium levels. Protein-energy malnutrition may affect various body functions and electrolyte balances, but it does not directly lead to increased serum potassium levels as prominently as Addison's Disease does.