A 59-year-old man presents with complaint of diffuse rash. He states he initially developed dry flaking scalp that 'came up overnight' and spread to involve his entire head about four weeks ago. He has been using over the counter pyrithione zinc shampoo with no improvement. About two weeks ago, he developed a bumpy red rash on the top of his fingers followed by a diffuse red rash on the chest, back, and abdomen. On exam, the temperature is 98.6 degrees Fahrenheit, blood pressure 118/78 mmHg, heart rate 80 beats per minute, respiratory rate 14 breaths per minute. There is a diffuse, well-demarcated red-orange plaque with prominent follicular hyperkeratosis on chest, abdomen, back, and extremities with intervening areas of normal skin and orange hyperkeratotic scale on palms and soles. What is the most appropriate first-line systemic treatment for this disease?