A previously healthy 8-year-old boy has a 3-week history of low-grade fever of unknown source, fatigue, weight loss, myalgia, and headaches. On repeated examinations during this time, he is found to have developed a heart murmur, petechiae, and mild splenomegaly. Which of the following is the most likely diagnosis?
a) Infective endocarditis
b) Acute rheumatic fever
c) Kawasaki disease
d) Systemic lupus erythematosus