A 28-year-old woman from Mumbai presents with secondary amenorrhea for 8 months. She reports a history of dilation and curettage (D&C) 10 months ago for incomplete abortion. She denies galactorrhea, hot flushes, or weight changes. On examination, she is normotensive and has normal secondary sexual characteristics. Pelvic examination is unremarkable. FSH is 45 mIU/mL (normal 4–12), LH is 38 mIU/mL (normal 2–8), and estradiol is <20 pg/mL (normal 30–400). Prolactin and TSH are normal. Pelvic ultrasound shows a normal uterus with thin endometrium (2 mm). What is the most likely diagnosis?