You are answering a USMLE-style medical question. Question: An obese 42-year-old female presents to her primary care physician with a complaint of left knee pain for the past 3 months. She describes the pain as affecting the inside part of the knee as well as the front of her knee, below the kneecap. She denies any inciting injury or trauma to the knee but reports that the pain is worse when she is climbing up stairs or rising from a chair. Physical examination is significant for localized tenderness to palpation over the left anteromedial proximal tibia, 6 cm inferior to the joint line. There is no joint effusion noted. Valgus stress testing is negative for any pain or instability of the knee joint. Radiographs of the left knee are obtained and reveal only mild arthritis, without evidence of any fractures or bony lesions. Which of the following is the most likely diagnosis in this patient? Options: A) Stress fracture of proximal tibia B) Medial meniscus tear C) Medical collateral ligament (MCL) sprain D) Pes anserine bursitis Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.