Prolonged fever with a distinct rectangular rash: What is going on?

We present the case of an older adult man with prolonged fever of unknown origin and a persistent rectangular erythematous plaque, highlighting the diagnostic value of careful skin examination in patients with systemic inflammatory symptoms. This 70-year-old man with adult-onset endogenous eczema, hypertension, stage II upper rectal carcinoma following resection and adjuvant radiotherapy and chemotherapy and left hip avascular necrosis presented with fever persisting for 1 month following an upper respiratory tract infection. He developed generalised arthralgia and bilateral lower limb oedema, accompanied by poor oral intake. Initial treatment with amoxicillin- clavulanate and intravenous piperacillin-tazobactam provided temporary relief. Physical examination revealed generalised xerosis, with an incidental finding of an erythematous, non-tender, mildly indurated, rectangular patch measuring 10x20 cm at the lower back, unnoticeable by the patient. Bilateral inguinal lymph nodes measuring 2x1 cm were palpable. Bilateral pedal oedema with joint bogginess was also present. Laboratory investigations showed leucocytosis with neutrophilia without blasts, normochromic normocytic anaemia, hypoalbuminaemia, transaminitis and elevated inflammatory markers. Imaging (computed tomography and positron emission tomography scans) revealed mild left pleural effusion with multiple subcentimetre lymphadenopathies above and below the diaphragm. Tuberculosis, malignancy and autoimmune markers were negative. Skin biopsy showed scattered single dyskeratotic cells at the upper epidermis, with mild-to-moderate mixed perivascular inflammation including lymphocytes, neutrophils and eosinophils at the upper dermis. Atypical cutaneous manifestations may serve as a valuable clue to adult-onset Still's disease; however, the diagnosis should be established only after careful exclusion of infectious, malignant and autoimmune mimics.