Final answer:
The patient with DIP tenderness, morning stiffness, deformity, dactylitis, and nail involvement most likely has psoriatic arthritis. Psoriatic arthritis is associated with skin changes and is characterized by symptoms less commonly seen in gout, osteoarthritis, or rheumatoid arthritis.
Step-by-step explanation:
The patient's symptoms of DIP tenderness, morning stiffness, deformity, dactylitis, and nail involvement are indicative of psoriatic arthritis (PsA). Psoriatic arthritis is strongly associated with skin psoriasis and often presents with nail changes and dactylitis, also known as 'sausage digits'. This condition also shows inflammation in the distal interphalangeal (DIP) joints, which is not as commonly seen in osteoarthritis (OA) or rheumatoid arthritis (RA), although RA can sometimes affect these joints.
Gout typically affects the metatarsophalangeal joints, especially the big toe, and would show more acute onset of symptoms alongside a characteristic tophaceous buildup, which differs from the more chronic and skin-involved pattern of psoriatic arthritis. Furthermore, morning stiffness and associated deformity are less characteristic of gout, which tends to have intermittent, rather than chronic, arthritic flares. Osteoarthritis, on the other hand, might present with joint deformity and some level of stiffness but is less likely to involve dactylitis and almost never features nail involvement. Rheumatoid arthritis is characterized by symmetrical polysynovitis and is more likely to involve proximal joints, such as the metacarpophalangeal (MCP), and can cause significant function impairment. The involvement of nail changes and dactylitis is less typical for RA. Therefore, answer (a) Psoriatic arthritis is the most likely diagnosis given the provided symptoms.