A 32-year-old woman presents with sudden-onset floaters, photopsia, and blurred vision in the right eye for 3 days. Examination reveals vitritis with 3+ haze, posterior synechiae, and a granulomatous anterior chamber reaction. Fundus examination shows multiple cream-coloured nodules at the macula and optic disc. Which investigation is most appropriate to establish the aetiology of this posterior uveitis?