A 16-year-old girl from Delhi presents with primary amenorrhea. She has normal secondary sexual characteristics, normal height (160 cm), and normal body habitus. Menarche has not occurred. On examination, she has normal breast development (Tanner stage 5) and normal pubic hair (Tanner stage 5). Pelvic ultrasound shows a normal uterus and ovaries with normal follicles. FSH and LH are normal, and prolactin is normal. Thyroid function tests are normal. What is the most likely diagnosis?