**Core Concept**
The evaluation of a 16-year-old girl presenting with rapid onset hirsutism and amenorrhea requires a thorough assessment of her hormonal profile, particularly focusing on androgen excess and ovarian function. This presentation is suggestive of hyperandrogenism, which can be caused by various conditions including polycystic ovary syndrome (PCOS), congenital adrenal hyperplasia (CAH), or androgen-secreting tumors.

**Why the Correct Answer is Right**
The most appropriate initial evaluation for this patient involves measuring serum levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone. This will help differentiate between PCOS, CAH, and other causes of hyperandrogenism. **Option A** is the correct answer because it involves assessing the patient's hormonal profile, which is essential for diagnosing the underlying cause of her symptoms.

**Why Each Wrong Option is Incorrect**
* **Option B:** Imaging studies such as ultrasound or CT scans may be useful in certain cases but are not the initial step in evaluating hirsutism and amenorrhea.
* **Option C:** Pregnancy testing is essential in reproductive-aged women presenting with amenorrhea but does not address the underlying cause of hirsutism.
* **Option D:** A thyroid function test may be useful in evaluating secondary amenorrhea but is not directly relevant to the evaluation of hirsutism.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the initial evaluation of hyperandrogenism should focus on assessing the patient's hormonal profile, including FSH, LH, and testosterone levels. This will help guide further diagnostic testing and treatment.

**Correct Answer:** A.