A 52-year-old man with a 10-year history of plaque psoriasis presents with painful swelling of his right index and middle finger for the past 2 months. Physical examination reveals dactylitis of the digit and nail pitting. Laboratory studies show negative rheumatoid factor and antinuclear antibodies. Radiographs of the hands demonstrate marginal erosions and new bone formation at the distal interphalangeal joints. Which of the following is the most appropriate initial disease-modifying therapy for this patient’s underlying condition?