A 28-year-old woman from Mumbai presents with secondary amenorrhea for 8 months. She has a history of dilation and curettage (D&C) 1 year ago for termination of pregnancy. On examination, she has normal secondary sexual characteristics and normal external genitalia. Serum FSH, LH, estradiol, and prolactin are all normal. Pelvic ultrasound shows a thin endometrium (< 4 mm) with a normal uterine cavity outline. What is the most appropriate next step in management?