A 21-year-old woman is evaluated in the outpatient clinic for increasingly irregular menst… — USMLE Step 2 CK

A 21-year-old woman is evaluated in the outpatient clinic for increasingly irregular menstrual periods and bothersome facial hair growth. Over the last 18 months, she has only had three menstrual bleeding episodes. She is frustrated by the recent onset of thick, dark hair on her chin and chest. Her medical history is unremarkable, and she takes no medications. Her body mass index is 29 kg/m². Her blood pressure and other vital signs are normal. Physical examination confirms the presence of hirsutism and acne. There is no evidence of temporal balding, deepening of the voice, or clitoral enlargement. A point-of-care urine pregnancy test is negative. The clinician suspects polycystic ovary syndrome but wishes to exclude other conditions that can present similarly. In addition to checking thyroid-stimulating hormone and prolactin levels, which of the following is the most appropriate next laboratory test to order for this patient?

A
Karyotype analysis
B
Luteinizing hormone to follicle-stimulating hormone ratio
Early morning serum 17-hydroxyprogesteroneAnswer
D
Overnight dexamethasone suppression test
E
Serum anti-Müllerian hormone

Answer

The correct answer is early morning serum 17-hydroxyprogesterone.

This patient's presentation of oligomenorrhea and clinical hyperandrogenism is highly suggestive of polycystic ovary syndrome (PCOS). However, PCOS is fundamentally a diagnosis of exclusion. Current clinical guidelines dictate that other disorders which mimic this presentation—such as thyroid dysfunction, hyperprolactinemia, and non-classic congenital adrenal hyperplasia (NCCAH)—must be explicitly ruled out. NCCAH, typically caused by a partial 21-hydroxylase deficiency, can present identically to PCOS with late-onset hyperandrogenism and ovulatory dysfunction. Therefore, measuring a fasting, early morning 17-hydroxyprogesterone level is an essential mandatory step in the initial diagnostic evaluation.

Step-by-Step Solution

Identify the patient's primary clinical syndrome based on her signs and symptoms.

The patient has oligomenorrhea (3 periods in 18 months) and clinical hyperandrogenism (hirsutism, acne), with a negative pregnancy test.

Establishing the core clinical presentation is the first step in formulating a differential diagnosis.

Determine the most likely suspected diagnosis and the criteria required to confirm it.

The most likely diagnosis is polycystic ovary syndrome (PCOS). However, PCOS is a diagnosis of exclusion, and mimicking conditions must be ruled out.

Adhering to standard diagnostic guidelines ensures that other treatable or distinct conditions are not missed.

Select the appropriate laboratory test to rule out the most common condition that mimics PCOS.

An early morning serum 17-hydroxyprogesterone level should be ordered to rule out non-classic congenital adrenal hyperplasia.

Non-classic congenital adrenal hyperplasia presents identically to PCOS and must be biochemically excluded to secure the diagnosis of PCOS.

Key Concept

The diagnosis of polycystic ovary syndrome (PCOS) requires the exclusion of other hyperandrogenic conditions; therefore, evaluating for non-classic congenital adrenal hyperplasia with an early morning 17-hydroxyprogesterone level is a mandatory part of the initial workup.

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