Part 1:~A population-based survey of Danish inhabitants, will by screening of approximately 10.000 Danes identify approximately 425 persons who report to have psoriasis(PsO) with or without psoriatic arthritis (PsA). These will receive an e-mail invitation to an internet based questionnaire regarding demographics, skin and joint complaints, diagnosed diseases, contact to health care providers, and different aspect of psychological and physical function and wellbeing (incl. function, health-related quality of life, depression, anxiety, social participation, and sleep disturbances). In the questionnaire the participant will be asked if he/she would be interested in participating in a clinical study.~Part 2:~Participants who accept the above mentioned invitation (estimated 273) will be seen in a Department of Rheumatology, for the following examination programme: Clinical examination with a focus on skin, joints and entheses,ultrasonic (US) examination of joints and entheses, patient-reported outcomes and blood sampling for both stratification and identification of biochemical signs of inflammation.~Patients with musculoskeletal pain and certain joint and/or entheseal inflammation documented by US, will be invited to participate in a 12 months' interventional study (part 3a, below), whereas patients without musculoskeletal pain but with US findings (as above) will be invited to participate in a 12 months non-interventional follow-up study (part 3b, below). Patients with pre-diagnosed PsA that by US have active inflammation (same definition and criteria as above), will also be invited to participate in the interventional study if they fit the criteria, especially those described under concomitant medication, otherwise they will be offered to participate in the non-interventional study.~Part 3a:~Patients with musculoskeletal pain in relation to joints and/or entheses (that is not explained by alternative diagnosis, as assessed by including rheumatologist) and US-defined PsA, i.e. with certain joint and/or entheseal inflammation as documented by US, will be offered inclusion in a 12 months' interventional study, in which 6 month induction therapy with apremilast (in addition to their usual therapy) will be followed by cessation of apremilast and 6 months of observation. Patients will be followed with clinical examination, PRO's, blood sampling and US at months 3, 6, 9 and 12.~MRI will be performed at inclusion and at 6 months follow-up in selected patients (patients with dactylitis or with enthesitis in the ankle region (Achilles enthesitis or plantar fasciitis)).~Part 3b:~Patients without musculoskeletal pain but with certain joint or entheseal inflammation verified by US will be offered inclusion in a 12 months' non-interventional study. Patients will continue their current therapy and be followed with clinical examination, patient-reported outcomes, blood sampling and US at months 3, 6, 9 and 12.
3-part study of patients with psoriasis, including 1) a population based questionnaire 2) cross-sectional clinical study with focus on musculoskeletal ultrasound and patient reported outcomes 3) 12 months follow-up study of patients with certain ultrasonic signs of psoriatic arthritis. Patients with pain: Interventional with 6 months treatment with apremilast, followed by 6 months observation. Patients without pain: 12 months observation.