A 37-year-old male had swelling and pain in the finger joints of both hands 2 years ago, which resolved with symptomatic treatment. Although arthralgia persisted, he continued working without systematic evaluation or treatment. Two months ago, he developed fever (~38°C), fatigue, and progressive shortness of breath with minimal cough. Examination revealed spindle-shaped swelling of the proximal interphalangeal joints of both middle fingers, slightly impaired function and tenderness in both wrists, inability to fully extend the right elbow joint, and a 1cm×1cm subcutaneous nodule on its extensor surface. Chest X-ray showed moderate pleural effusion on the right side, small pleural effusion on the left side, and increased pulmonary markings. ESR was 56mm/1st hour, CRP elevated, and rheumatoid factor (RF) strongly positive. Which finding most suggests the diagnosis?