You are answering a USMLE-style medical question. Question: A 55-year-old man recovering from knee replacement surgery complains of breathlessness in the postoperative ward. He has been confined to bed for the past 5 days and is under observation. He felt a sudden difficulty in breathing and called for the ward nurse. He says that he is unable to take deep breaths and has a sharp pain on the right side of his chest with each inspiration. His temperature is 37.5°C (99.8°F), the pulse is 111/min, the respirations are 31/min, and the blood pressure is 85/55 mm Hg. He experiences pain in his right calf on dorsiflexion. There are no other prominent findings on physical examination. His chest X-ray does not show anything significant. The ECG reveals sinus tachycardia. Which of the following is the best course of management at this time? Options: A) Subcutaneous fondaparinux B) Inferior vena cava filter C) Ventilation-perfusion scan D) Lower extremity doppler Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.