Menarche in the third decade: congenital adrenal hyperplasia masquerading as Mayer-Rokitansky-Küster-Hauser syndrome.

Primary amenorrhea (PA) in a young girl can result from structural, functional, or hormonal disorders. Accurate diagnosis is essential for counseling on hormone replacement and fertility. Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome is characterized by an absent or hypoplastic uterus and presents with PA. Congenital adrenal hyperplasia (CAH) is a steroidogenic defect that may present at any age but rarely as PA. We report a 33-year-old woman who initially presented at age 17 with PA and was labeled as having MRKH syndrome due to a hypoplastic uterus. She was treated by multiple dermatologists for resistant acne between ages 20 and 32. During evaluation of hyperandrogenism, her computed tomography scan of the abdomen revealed a right adrenal mass, prompting an endocrine referral. Our evaluation showed virilization, clitoromegaly, and a hormonal profile suggestive of simple virilizing CAH. Gene sequencing showed a homozygous missense variation in exon 4 of the CYP21A2 gene (chr6:g.32007203T>A) (c.518T>A) (p.Ile173Asn), confirming CAH. She was managed with glucocorticoids, antiandrogens, and estrogen supplements. She had menarche 6 months after therapy at age 33. We report this case to highlight the importance of detailed endocrine evaluation in patients with PA.