• Placebo to Guselkumab 1 mL liquid formulation in a single-dose pre-filled syringe administered by subcutaneous injection at Week 0, Week 4 and every 8 weeks thereafter (Month 0 to Week 20 for Arm 2).

To meet CASPAR criteria for diagnosis of PsA, a participant must have inflammatory articular disease (joint, spine, entheseal or dactyitic) and at least 3 points from the following:

1. Evidence of current psoriasis (2pts), personal history of psoriasis (2pts), family history (1pt)
2. Typical psoriatic nail dystrophy including onycholysis, pitting, and hyperkeratosis observed on current physical examination (1pt)
3. A negative test result for the presence of rheumatoid factor by any method except latex (1pt)
4. Either current dactylitis, defined as swelling of an entire digit, or a history of dactylitis recorded by a rheumatologist (1pt)
5. Radiographic evidence of juxta-articular new bone formation appearing as ill-defined ossification near joint margins (but excluding osteophyte formation) on plain radiographs of the hand or foot (1pt) The eCRF will autopopulate the total number of points. If the total score ≥ 3, the participant meets criteria for PsA diagnosis.

Severity will be categorized as mild, moderate, or severe.

Graded from 0-3 as absent, mild, moderate or severe according to images of a reference atlas (Hammer HB 2011). PD signal: 0=no PD-signal, 1=up to three single or two confluent signals, 2=less than half of the visible intracapsular area and 3=half or more of the visible intracapsular area covered by PD-signals.

MASEI: Structure is considered pathological (score=1) if there is a loss of fibrillar pattern, hypoechoic aspect, or fusiform thickening of the entheses. Erosions are defined as a cortical breakage with a step-down contour defect at the attachment of entheses at bone and graded with 0=absent or 3=present. Fascia and tendon thickness are measured at the point of maximal thickness on the bony insertion and graded with 0=normal or 1=thickened according to the reference values of the MASEI index. Enthesophytes are defined as calcifications at the entheses insertions into bone and graded with 0=absent, 1=small calcification, 2=clear presence of enthesophyte/calcification, 3= large calcifications or ossifications. PD-signals within entheses are scored with 0=absent or 3=present. Bursitis is investigated at the level of distal patellar tendon (infrapatellar bursitis) and the level of Achilles tendon insertion (retrocalcaneal bursitis) and graded with 0=absent and 1=present.

The total BSA affected by plaque-type psoriasis will be estimated from the percentages of areas affected, including head, trunk, upper limbs and lower limbs. The following calculations will be done: each reported percentage will be multiplied by its respective body region corresponding factor (head = 0.1, trunk = 0.3, upper limbs = 0.2, lower limbs = 0.4). The resulting four percentages will be added up to estimate the total BSA affected by psoriasis.

Score 0 - Clear - No signs of psoriasis. Post-inflammatory hyperpigmentation may be present Score 1 - Almost clear - Normal to pink coloration of lesions; no thickening; no to minimal focal scaling Score 2 - Mild disease - Pink to light red coloration; just detectable to mild thickening; predominantly fine scaling Score 3 - Moderate disease - Dull bright red, clearly distinguishable erythema; clearly distinguishable to moderate thickening; moderate scaling Score 4 - Severe disease - Bright to deep dark red coloration; severe thickening with hard edges; severe / coarse scaling covering almost all or all lesions

FACIT consists of 13 statements regarding fatigue (e.g., "I feel fatigued", "I feel weak all over", "I feel tired", etc.). Items are scored as follows: 4=not at all, 3=a little bit, 2=somewhat, 1=quite a bit; 0=very much, EXCEPT items #7 and 8 which are reversed scored. Total score range is 0-52. A score of less than 30 indicates severe fatigue. The higher the score, the better the quality of life.

The scale measures how good or bad one's health is on the day of the questionnaire. Total score is 0-100; the higher the score, the better the health (0 = worst health one can imagine, 100 = best health one can imagine)

The scale measures how good or bad one's health is on the day of the questionnaire. Total score is 0-100; the higher the score, the better the health (0 = worst health one can imagine, 100 = best health one can imagine)

IDEOM-MSK-8 is a short questionnaire that allows people with MSK conditions to report their symptoms and quality of life in a standardized way. The total range of score is 0-56; the higher the score, the better the MSK health status. In order to calculate the total score, the numbers next to the boxes that the participant has ticked on the questionnaire form is added up.

Score is defined by the ultrasound (General Electric Logiq E9 or E10) equipment, not calculated through a scale.

Assessed using ultrasound results.