Which of the following is a characteristic feature of psoriatic arthritis?

Psoriatic arthritis is a type of inflammatory arthritis associated with the skin condition psoriasis. One of its hallmark features is the involvement of the distal interphalangeal (DIP) joints, which is a distinguishing factor compared to other forms of arthritis, such as rheumatoid arthritis. Dactylitis, characterized by the swelling of an entire digit (a fingertip or toe, resembling a "sausage"), is commonly seen in psoriatic arthritis. This combination of DIP joint involvement and dactylitis is particularly characteristic and helps differentiate psoriatic arthritis from other types of inflammatory arthritis.

Other options mentioned relate to different types of arthritis or do not accurately capture the unique features of psoriatic arthritis. Symmetrical polyarthritis is more typical of rheumatoid arthritis, where joint involvement is often symmetric and more generalized across the body. Morning stiffness can be present in various forms of inflammatory arthritis, including psoriatic arthritis, but it is not as distinctive a feature as DIP involvement with dactylitis. Swan-neck deformities are typically associated with rheumatoid arthritis and not psoriatic arthritis. Thus, the involvement of the DIP joints along with dactylitis is the most distinguishing feature of psoriatic arthritis.