This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vasculitis, Subject name: Pathology
A 30-year-old woman with Sjogren syndrome presents with a 24-hour history of a purpuric skin rash. Which of the following is the most likely diagnosis? 
Select and explain. 
A. Buerger disease, B. Giant cell granulomatous aeritis, C. Hypersensitivity vasculitis, D. Wegener granulomatosis
C: Hypersensitivity vasculitis
Answer: C: Hypersensitivity vasculitis 
Explanation: Purpura or skin rash in a patient with a known autoimmune disease, such as Sjogren syndrome or systemic lupus erythematosus, is usually attributed to hypersensitivity vasculitis. This vasculitis is caused by the deposition of immune complexes in dermal venules. The other choices are not primarily immune complex diseases.