A married couple visits a reproductive endocrinology specialist to discuss their inability… — USMLE Step 2 CK

A married couple visits a reproductive endocrinology specialist to discuss their inability to conceive over the past $14$ months. The female partner, who is $30$ years old, reports that her menstrual periods ceased entirely $8$ months ago. Prior to this, her cycles occurred predictably every $28$ days. Recently, she has been troubled by disrupted sleep, sudden episodes of warmth over her chest, and dyspareunia. Her past medical history is unremarkable. Family history is significant for her sister's $6$ -year-old son, who was recently diagnosed with global developmental delay, autism spectrum disorder, and large protruding ears. The patient's body mass index is $22 \text{ kg/m}^2$ . Pelvic examination shows thin, pale vaginal mucosa. A urine pregnancy test is negative. Laboratory studies demonstrate a serum follicle-stimulating hormone (FSH) level of $72 \text{ mIU/mL}$ and a luteinizing hormone (LH) level of $45 \text{ mIU/mL}$ .

Which of the following pathophysiologic mechanisms best explains this patient's current condition?

Premutation expansion of a CGG trinucleotide repeat sequenceAnswer
B
Altered pulsatility of gonadotropin-releasing hormone with hyperinsulinemia
C
Reversible suppression of the hypothalamic-pituitary-ovarian axis
D
Ischemic necrosis of the anterior pituitary gland
E
Meiotic nondisjunction resulting in an absent sex chromosome

Answer

The correct pathophysiologic mechanism is the premutation expansion of a CGG trinucleotide repeat sequence.

This patient presents with secondary amenorrhea, infertility, and symptoms of estrogen deficiency (vasomotor symptoms, dyspareunia, thin vaginal mucosa). Her markedly elevated follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels confirm a diagnosis of hypergonadotropic hypogonadism, which in a woman under age 40 defines primary ovarian insufficiency (POI). The critical clue to the underlying etiology is her family history of a nephew with developmental delay, autism, and large ears, which is the classic presentation of Fragile X syndrome. Fragile X syndrome is caused by a full mutation (>200 CGG repeats) in the FMR1 gene. Females who carry the premutation (55-200 CGG repeats) do not typically have intellectual disability but are at a roughly 20% risk of developing FMR1-related POI. The premutation leads to the toxic accumulation of FMR1 mRNA, causing accelerated apoptosis of ovarian follicles and the early cessation of menses.

Step-by-Step Solution

Identify the primary clinical syndrome based on the patient's symptoms and laboratory results.

The combination of secondary amenorrhea, vasomotor symptoms (hot flashes), vaginal atrophy, and markedly elevated FSH/LH indicates hypergonadotropic hypogonadism, specifically primary ovarian insufficiency (POI).

Elevated gonadotropins in the setting of clinical estrogen deficiency confirm that the defect lies at the level of the ovary (loss of negative feedback) rather than the hypothalamus or pituitary.

Analyze the provided family history to determine the specific etiology of the POI.

The nephew's classic triad of developmental delay, autism spectrum disorder, and large protruding ears is highly characteristic of Fragile X syndrome.

Connecting the patient's ovarian failure to her sister's child's genetic disorder isolates the exact molecular mechanism responsible for her infertility.

Correlate Fragile X genetics with female ovarian function.

The patient is likely a carrier of the FMR1 premutation (55-200 CGG trinucleotide repeats).

Female carriers of the FMR1 premutation have a 20% risk of developing POI due to accelerated follicular depletion, perfectly explaining her clinical presentation.

Key Concept

Primary Ovarian Insufficiency (POI) associated with the Fragile X (FMR1 gene) premutation.

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