A 32-year-old woman from Mumbai presents with primary infertility of 4 years. She reports menarche at age 14, regular menstrual cycles until age 28, then progressive oligomenorrhea (cycles now 45–60 days apart). She denies hot flushes, night sweats, or mood changes. On examination, BMI 22 kg/m², normal secondary sexual characteristics, no galactorrhea. Transvaginal ultrasound shows small, fibrotic ovaries with few or no visible follicles. Serum FSH 68 mIU/mL, LH 52 mIU/mL, estradiol <20 pg/mL, anti-Müllerian hormone (AMH) <0.5 ng/mL. Karyotype is 46,XX. What is the most likely diagnosis?