You are answering a USMLE-style medical question. Question: A 35-year-old woman comes to the physician for sleeping problems and the inability to concentrate for 3 months. She says she is worried because she and her husband have been trying to conceive for more than a year with no success. Conception by in vitro fertilization was attempted once 3 months ago but was unsuccessful. Analysis of her husband's semen has shown normal sperm counts and morphology. She has a 6-year-old daughter who was born at term after an uncomplicated pregnancy. She has no history of severe illness and tries to stay healthy by going to the gym an hour per day. Her menses occur at regular 28-day intervals and last 5 to 6 days; her last menstrual period started 2 days ago. Physical examination shows no abnormalities. Which of the following is the most likely cause of this patient's inability to conceive? Options: A) Pelvic inflammatory disease B) Diminished ovarian reserve C) Hypogonadotropic hypogonadism D) Premature ovarian failure Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.