Which of the following is considered a major criterion in the Duke criteria?

The major criterion in the Duke criteria for diagnosing infective endocarditis is the presence of two positive blood cultures with specific organisms. This criterion is essential because it demonstrates the presence of bacteremia caused by organisms that are known to be associated with infective endocarditis, such as Streptococcus viridans and Staphylococcus aureus. The identification of these organisms in blood cultures helps confirm the diagnosis and is critical for appropriate management and treatment.

The other options, while relevant to the diagnosis of infective endocarditis, do not meet the criteria for major classification. Fever is common but not unique or definitive for endocarditis, recent dental procedures can provide context but are not a direct indicator, and the presence of Osler's nodes represents a minor criterion rather than a major one. Thus, the identification of specific organisms in blood cultures stands out as a foundational element in the diagnosis according to the Duke criteria.