A 28-year-old woman presents with intensely pruritic, grouped vesicles on her elbows and knees that have been present for several months. She also reports intermittent diarrhea, bloating, and a 5-kg weight loss over the past year. Laboratory studies reveal iron deficiency anemia and positive anti–tissue transglutaminase IgA antibodies. A skin biopsy with direct immunofluorescence shows granular IgA deposition in dermal papillae. What is the most appropriate initial management for her rash and gastrointestinal symptoms?