## **Core Concept**
The clinical presentation of rapid onset hirsutism and amenorrhea in a 16-year-old girl suggests an androgen excess disorder. The most common causes include polycystic ovary syndrome (PCOS), congenital adrenal hyperplasia, androgen-secreting tumors, and idiopathic hyperandrogenism. Given the rapid onset, a more sinister cause such as an androgen-secreting tumor needs to be ruled out promptly.

## **Why the Correct Answer is Right**
The best investigation for rapidly progressive hirsutism and amenorrhea, particularly when suspecting an androgen-secreting tumor, is **D. Serum testosterone level**. This is because androgen-secreting tumors can cause a significant and rapid increase in androgen levels, leading to symptoms like hirsutism and amenorrhea. Measuring serum testosterone levels can help identify such conditions. A very high level of testosterone is suggestive of an ovarian or adrenal tumor.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While ultrasound can be useful in evaluating ovarian morphology and detecting ovarian tumors, it does not directly assess the hormonal cause of symptoms.
- **Option B:** LH and FSH levels can help diagnose PCOS and assess the hypothalamic-pituitary-gonadal axis but are not the best initial test for rapidly progressive symptoms suggestive of a tumor.
- **Option C:** Pelvic examination and ultrasound might be part of the evaluation but are not the best initial investigations for the acute presentation of hirsutism and amenorrhea.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is to consider an androgen-secreting tumor in cases of rapid onset hirsutism, especially if accompanied by other virilizing symptoms. A significantly elevated serum testosterone level (>150 ng/dL) is highly suggestive of a tumor, and such patients require prompt imaging studies to localize the tumor.

## **Correct Answer:** . Serum testosterone level