**Core Concept:** Oligomenorrhea and amenorrhea are conditions characterized by irregular or absent menstrual periods, respectively. They may be indicative of underlying hormonal imbalances, endocrine disorders, or other medical conditions. Blood pressure assessment provides important information about cardiovascular health.

**Why the Correct Answer is Right:** In this scenario, the patient presents with oligomenorrhea and amenorrhea along with high blood pressure. The most appropriate investigation to determine the cause of these symptoms would be an adrenal function test (Option D). This is because oligomenorrhea and amenorrhea can be related to disorders affecting the hypothalamic-pituitary-adrenal (HPA) axis, leading to secondary adrenal insufficiency. High blood pressure may also be a manifestation of this condition.

**Why Each Wrong Option is Incorrect:**

A. A thyroid function test is appropriate when hypothyroidism is suspected, which is not mentioned in the question.
B. A liver function test is relevant when liver disease or dysfunction is suspected, which is not the primary concern in this scenario.
C. A general physical examination is important, but it is not a specific test to investigate the patient's symptoms.

**Why Each Wrong Option is Incorrect:**

A. A thyroid function test is appropriate when hypothyroidism is suspected, which is not mentioned in the question.
B. A liver function test is relevant when liver disease or dysfunction is suspected, which is not the primary concern in this scenario.
C. A general physical examination is important, but it is not a specific test to investigate the patient's symptoms.

**Clinical Pearl:** It is essential for medical students and practitioners to consider the possibility of adrenal insufficiency in patients presenting with irregular periods and high blood pressure, especially in the absence of other obvious causes. A comprehensive workup should include an adrenal function test to investigate the primary symptoms and potentially address the secondary hypertension.