PLAB Endocrinology Practice Exam

The condition indicated by a serum potassium level of 2.5mmol/L, particularly in the context of secondary amenorrhea and severe acne, is more consistent with Cushing's syndrome. In Cushing's syndrome, there is an excess of cortisol, which can lead to various metabolic disturbances. One common effect of high cortisol levels is the promotion of sodium retention and potassium excretion by the kidneys, often resulting in hypokalemia, which is a low potassium level in the serum.

Secondary amenorrhea and severe acne can also be seen in Cushing's syndrome: the hormonal changes due to excessive cortisol can disrupt the normal menstrual cycle and increase androgen levels, leading to the development of acne. Therefore, the combination of these symptoms, particularly alongside a significantly low potassium level, points towards Cushing's syndrome as the underlying condition.

In contrast, conditions like primary hyperaldosteronism typically cause low potassium but are more often associated with hypertension and other signs related to aldosterone excess. Addison's disease would more commonly present with hyperkalemia (high potassium), and acquired hypothyroidism does not typically result in significant changes in potassium levels or the symptoms described. Thus, Cushing's syndrome aligns with the clinical picture.