The most common extra-articular manifestation of rheumatoid arthritis is:

The most common extra-articular manifestation of rheumatoid arthritis is rheumatoid nodules. These nodules are firm, painless subcutaneous lumps, typically found over pressure points such as the elbows and fingers. They represent granulomatous inflammation and are strongly associated with seropositive RA, especially in patients with high rheumatoid factor levels. Unlike other manifestations, rheumatoid nodules are a distinguishing feature directly linked to the disease process and are more prevalent than cardiac or ocular complications. Recognizing these nodules aids in clinical diagnosis and assessment of disease severity.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.