A 32-year-old woman from Mumbai presents with a 6-month history of progressive muscle weakness, ptosis, and diplopia. Clinical examination suggests myasthenia gravis (MG). Serology for anti-acetylcholine receptor (AChR) antibodies is negative, but clinical suspicion remains high. The neurologist suspects seronegative MG and wants to confirm the diagnosis. Which molecular technique should be used next to detect anti-muscle-specific kinase (MuSK) antibodies and differentiate this subset?