What would be your next step in management of a dermatomyositis patient on hydroxychloroquine and methotrexate whose skin is still active?

There are many different options for this kind of scenario. Depending on the physician's preference, patient's lifestyle, and insurance coverage, one could switch to mycophenolate, which seems to be more effective for skin manifestations, or azathioprine.

If the patient did have a partial response to methotrexate, one could add azathioprine in addition to methotrexate; the benefit of this option is that the patient does not require any bridging, but on the other hand, this regimen includes 2 medications.

As a side note, while hydroxychloroquine is essential for lupus manifestations, up to 25% of patients with dermatomyositis might develop an adverse reaction. So, one consideration would be to stop the hydroxychloroquine if the rash worsened...