The most likely diagnosis is rheumatoid arthritis (RA), which is characterized by a symmetric inflammatory polyarthritis of the small joints. Autoantibodies such as rheumatoid factor or anti–cyclic citrullinated peptide (CCP) antibodies may be present, although autoantibodies are neither necessary nor sufficient for diagnosis. Anti-CCP antibodies occur less frequently than rheumatoid factor, but their presence has more diagnostic specificity for RA. Some patients with RA also lack rheumatoid factor. Seronegative RA has an identical clinical appearance as seropositive RA but is more likely to occur in men. Despite a negative rheumatoid factor and anti-CCP antibodies, this patient's clinical presentation of polyarticular inflammatory arthritis involving multiple and bilateral interphalangeal joints of the fingers, metacarpophalangeal joints, a wrist, and an ankle as well as prolonged morning stiffness and radiographic findings of marginal erosion and periarticular osteopenia, is characteristic of RA. Over time, some patients who are initially seronegative develop a positive rheumatoid factor.

This patient does not have monoarticular or oligoarticular disease or radiographs showing bony sclerosis or osteophyte formation, all of which are typical of osteoarthritis. This patient's symmetric polyarticular inflammatory arthritis associated with prolonged morning stiffness is not consistent with osteoarthritis, in which joint swelling is not found and morning stiffness lasts less than 30 minutes.

Although sarcoidosis can occasionally cause joint involvement, it is unlikely to present with joint symptoms alone. Chronic sarcoid arthropathy most commonly involves the ankles, knees, hands, wrists, and metacarpophalangeal and proximal interphalangeal joints and is usually accompanied by parenchymal pulmonary disease. It is unlikely to be the cause of inflammatory polyarthritis in a previously healthy middle-aged man.

Although systemic lupus erythematosus (SLE) can cause seronegative polyarticular inflammatory arthritis, the initial presentation in a middle-aged man as an explanation for polyarticular inflammatory arthritis would be exceedingly unlikely, and erosions are not seen as a result of arthritis in SLE. The patient has no signs or symptoms otherwise suggestive of SLE such as brain, kidney, lung, heart, or skin manifestations.