The patient’s presentation with fever, malar rash, joint pain, oral ulcers, and photosensitivity is characteristic of Systemic Lupus Erythematosus (SLE). SLE is a multisystem autoimmune disease with diverse manifestations, including the classic malar (butterfly) rash sparing the nasolabial folds. The best initial serologic test is the antinuclear antibody (ANA), which is highly sensitive but not specific. To confirm the diagnosis and assess disease activity, anti-double stranded DNA (anti-dsDNA) antibodies are measured, as they are more specific to SLE and correlate with disease severity, especially lupus nephritis. This combination guides diagnosis and management effectively.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 21st Edition.