What is the most likely cause of the fevers?

A 30-year-old man presents with a 4-week history of fevers (as high as 40°C), night sweats, and swelling of the right side of the neck. He did not respond to an empiric course of antibiotics. His fever curve and response to acetaminophen and a nonsteroidal anti-inflammatory drug (NSAID) are shown. What is the most likely cause of the fevers?

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a. Abscess
b. Endocarditis
c. Neoplasm
d. Medication allergy
e. Chronic viral infection