A 75-year-old man is admitted to the hospital because of community acquired pneumonia and worsening of his heart failure. He takes lisinopril and metoprolol. On examination, he is febrile, BP 110/65 mmHg, HR 90/min, and respiratory rate 24/min. He has leukocytosis and signs of right lower lobe consolidation on chest x-ray Which of the following is the most appropriate VTE prophylaxis in this patient?