A 48-year-old woman from Odisha with borderline lepromatous leprosy, 3 months into multidrug therapy, presents with acute swelling and tenderness of the right ulnar nerve, along with sudden loss of sensation in the 4th and 5th fingers. She also reports pain and redness in her existing facial lesions. Temperature is 37.2°C. Slit-skin smear shows moderate acid-fast bacilli. What is the most likely diagnosis, and which nerve complication is most characteristic of this reaction?