A 38-year-old man with a history of intravenous drug use presents with fever, new cardiac murmur, and positive blood cultures for Staphylococcus aureus. A 42-year-old woman with a prosthetic mitral valve presents with fever, changing murmur, and positive blood cultures for Streptococcus viridans. Which single feature best distinguishes native valve endocarditis from prosthetic valve endocarditis?