- Phase I: Patients will be included once they are started on systemic fumaric acid esters (e.g. Fumaderm initial®) for the treatment of moderate-to-severe psoriasis vulgaris. Patients initiated on fumaric acid esters will be followed for a maximum of 24 weeks during phase I. Patients continued on fumaric acid esters beyond 24 weeks or switched to a therapy other than apremilast during or at the end of the 24-week phase I period will no longer be followed.