A 28-year-old woman from Kerala presents to the emergency department on day 4 of illness with fever, severe retro-orbital pain, myalgia, and a petechial rash on her trunk and extremities. Her platelet count is 45,000/μL, haemoglobin 16.5 g/dL, and haematocrit 48%. She denies arthralgia. Tourniquet test is positive. What is the most likely diagnosis?