A 28-year-old Nigerian woman presents to a UK hospital with a 5-day history of fever, cough, and pleuritic chest pain 2 weeks after returning from visiting family in Lagos. Chest X-ray shows bilateral patchy infiltrates. She is HIV-negative. Malaria RDT is negative. Blood cultures are negative. She reports a rat infestation at the family home. Serology returns positive for Lassa virus IgM. What is the most appropriate treatment?