Step 1: The image shows a photosensitive, raised, erythematous malar (butterfly) rash across the cheeks and bridge of the nose, the most characteristic cutaneous sign of systemic lupus erythematosus (SLE).
Step 2: The malar rash develops at some point in 55-85% of SLE patients and is triggered or worsened by sun exposure. SLE may also show oral, nasal or vaginal ulcers and alopecia in about 50% of cases.
Step 3: Discoid lupus (c) instead gives circular, scaly, hyperpigmented plaques with an erythematous rim and atrophic scarring center, seen in 15-30%, and is localized rather than a malar pattern.
Step 4: Polymorphous light eruption (b) is an itchy papular sun-induced rash without systemic features, and skin tuberculosis (d) presents as chronic plaques or ulcers (lupus vulgaris), neither matching the classic malar distribution shown.