A 35-year-old man of Northern European descent presents with a 6-month history of intensely pruritic grouped vesicles that he has been scratching, leaving crusted erosions. On examination, the lesions are symmetrically distributed on the elbows, knees, buttocks, and scalp—as marked by **C** in the diagram. The patient reports no significant gastrointestinal symptoms. Skin biopsy with direct immunofluorescence shows granular IgA deposits at the tips of dermal papillae. Which of the following is the most appropriate next diagnostic step to confirm the underlying systemic disease associated with this cutaneous manifestation?