A key diagnostic feature of rheumatic fever is the presence of Aschoff’s bodies in cardiac tissue.
Aschoff’s bodies are nodules formed due to fibrinoid necrosis surrounded by immune cells — they are pathognomonic for rheumatic carditis.
Antischkow cells are modified macrophages found within Aschoff bodies but by themselves are not specific diagnostic markers.
MacCallum’s patch refers to areas of fibrosis in the left atrium but is not a primary diagnostic feature.
Epithelioid cells are seen in granulomatous diseases like tuberculosis, not rheumatic fever.
Therefore, the diagnostic hallmark of rheumatic fever is Aschoff’s bodies.