**Core Concept**
The patient's presentation of weight gain, oligomenorrhea followed by amenorrhea, and hypertension is suggestive of Cushing's syndrome, a condition caused by excess cortisol levels in the body. This condition can be due to various etiologies, including endogenous or exogenous sources of cortisol.

**Why the Correct Answer is Right**
Plasma cortisol levels are a direct measure of the hormone's concentration in the blood. In Cushing's syndrome, the cortisol levels are typically elevated due to an excess of cortisol production. Measuring plasma cortisol levels is a crucial step in diagnosing Cushing's syndrome. This test can help distinguish between endogenous and exogenous sources of cortisol. For example, if the cortisol levels are high in the morning (when cortisol levels should be low) and do not suppress after a low-dose dexamethasone suppression test, it suggests endogenous Cushing's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum electrolytes would not directly help in diagnosing Cushing's syndrome, although electrolyte imbalances can be a consequence of the disease.

**Option C:** While plasma testosterone and ultrasound evaluation of the pelvis may be helpful in evaluating secondary amenorrhea, they do not directly address the patient's hypertension and weight gain.

**Option D:** T3, T4, and TSH levels are used to evaluate thyroid function and are not directly relevant to the patient's presentation of weight gain, oligomenorrhea, and hypertension.

**Clinical Pearl / High-Yield Fact**
Cushing's syndrome can be caused by various etiologies, including endogenous (e.g., pituitary adenoma, adrenal adenoma) and exogenous (e.g., glucocorticoid use) sources of cortisol. A high index of suspicion and a thorough evaluation are necessary to diagnose this condition.

**✓ Correct Answer: B. Plasma cortisol**