You are answering a USMLE-style medical question. Question: A healthy 23-year-old male is undergoing an exercise stress test as part of his physiology class. If blood were to be sampled at different locations before and after the stress test, which area of the body would contain the lowest oxygen content at both time points? Options: A) Inferior vena cava B) Coronary sinus C) Pulmonary artery D) Pulmonary vein Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.