## **Core Concept**
The patient's presentation of significant weight gain, oligomenorrhea followed by amenorrhea, and high blood pressure suggests a condition affecting multiple endocrine systems. The symptoms point towards Cushing's syndrome, a disorder caused by excess levels of **cortisol** in the body.

## **Why the Correct Answer is Right**
The symptoms described—rapid weight gain, menstrual irregularities (oligomenorrhea leading to amenorrhea), and hypertension—are classic for Cushing's syndrome. The most appropriate investigation to confirm Cushing's syndrome would involve assessing cortisol levels or its effects. **Dexamethasone suppression test** is a key diagnostic tool; it checks the body's ability to regulate cortisol production. In Cushing's syndrome, cortisol levels remain elevated despite dexamethasone administration.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While imaging studies like CT or MRI might be useful in localizing an adrenal tumor (a potential cause of Cushing's syndrome), they are not the first line in diagnosing the condition itself.
- **Option B:** Although thyroid function tests can help diagnose thyroid issues that might cause weight gain and menstrual irregularities, they do not directly address the hypertension and the specific constellation of symptoms suggesting Cushing's syndrome.
- **Option C:** Lipid profiles can provide information on cholesterol levels, which might be affected by obesity and hypertension but do not help diagnose the underlying cause of the patient's symptoms.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is to suspect Cushing's syndrome in patients presenting with **central obesity**, **hypertension**, **glucose intolerance**, and **amenorrhea**. A simple screening test like the **1-mg overnight dexamethasone suppression test** can be a good starting point, where a failure to suppress cortisol production indicates potential Cushing's syndrome.

## **Correct Answer:** D. Dexamethasone suppression test.