ukmla MCQ (SBA): Pulmonary Embolism Diagnosis

A 35-year-old woman presents with acute shortness of breath and pleuritic chest pain. She recently returned from a long-haul flight (12 hours). On examination, she is tachycardic (110 bpm) and tachypnoeic (28/min). O2 saturation is 92% on air. Her Wells score for pulmonary embolism (PE) is 4.5 (clinically suspected PE, tachycardia, recent immobilization). What is the most appropriate next investigation?