A 32-year-old woman from Kerala presents with sudden onset of high-grade fever, severe joint pain (especially wrists, ankles, and knees), and a maculopapular rash on the trunk and limbs for 3 days. She also reports myalgia and headache. Clinical examination reveals arthralgia with mild swelling of the affected joints. What is the most appropriate investigation of choice to confirm the diagnosis at this stage of illness?