In a case of infectious mononucleosis the first and only symptoms were those of acute pericarditis. A careful diagnostic survey is most important in each instance of "nonspecific" idiopathic pericarditis, including serial heterophil tests for infectious mononucleosis.
Shapiro SC, Dimich I, Steier M. Pericarditis as the Only Manifestation of Infectious Mononucleosis. Am J Dis Child. 1973;126(5):662-663. doi:10.1001/archpedi.1973.02110190534015