You are answering a USMLE-style medical question. Question: A 53-year-old female patient comes to the physician’s office for her annual check-up visit. She has no complaints and her past medical history is notable for diabetes and hypertension. During this visit, the patient undergoes screening procedures per guidelines including a mammogram. On the screening mammogram a spiculated, irregular mass is found on the left breast. Further diagnostic mammography and biopsy reveal ductal adenocarcinoma of the breast in the upper outer quadrant of the left breast. Which of the following is the most important factor in determining this patient’s prognosis? Options: A) Tumor grade B) Tumor stage C) Age D) Location of the tumor on the breast Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.