Infectious causes (particularly Q fever) were specifically reconsidered: the serologic pattern (IgG phase II ≥1:200 with phase I ≥1:50 and negative IgM) is more consistent with past exposure or chronic infection rather than acute disease; however, there were no clinical or imaging features of chronic Q fever (e.g., endocarditis or vascular infection), and echocardiography was unremarkable. Here, CD40LG is linked to endocarditis.