A 37-year-old male had swelling and pain in the finger joints of both hands two years ago, which resolved with symptomatic treatment. Although he experienced arthralgia afterward, he could work normally without systematic examination or treatment. Two months ago, he developed fever (~38°C), fatigue, and gradually worsening shortness of breath with minimal cough. Physical examination: spindle-shaped swelling of the proximal interphalangeal joints of both middle fingers, slightly impaired function of both wrists with tenderness, inability to fully extend the right elbow joint, and a 1cm×1cm subcutaneous nodule on the 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/hr, CRP was elevated, and RF was high-titer positive. What is the most likely cause of bilateral pleural effusion in this patient?