A 28-year-old woman presents with a 2-week history of a solitary, asymptomatic, scaly patch on her left upper back. She reports mild pruritus. One week later, she develops a widespread eruption of small, oval, scaly papules and plaques on her trunk and proximal limbs, distributed along Langer's lines. The lesions are salmon-pink in colour. She denies fever, arthralgia, or recent upper respiratory infection. Serological testing for syphilis is negative. What is the most likely diagnosis?