A 58-year-old man with a 3-week history of deep vein thrombosis (DVT) of the left lower limb presents to the emergency department with acute onset chest pain and breathlessness. CT pulmonary angiography confirms pulmonary embolism (PE). He has normal renal function (eGFR 85 mL/min/1.73 m²) and no contraindications to anticoagulation. What is the drug of choice for initial anticoagulation in this patient?