MSRA-1204

A 45-year-old man presents to his General Practitioner with a 7-day history of flu-like symptoms, including spiking fevers, headache, rigors, vomiting and diarrhoea.
On examination, he looks unwell, is febrile and jaundiced. His abdomen is soft, with mild, generalised tenderness and hepatosplenomegaly. He returned from Nigeria six months ago after a 8-week stay with a charity mission. He had all the appropriate travel vaccinations and took mefloquine prophylaxis.
What is the most likely diagnosis?