This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Autoimmune Skin Disorders, Subject name: Skin
Which of the following is NOT a primary skin disease? 
Select and explain. 
A. Reiter's syndrome, B. Bowen's disease, C. Psoriasis, D. Lichen planus
A: Reiter's syndrome
Answer: A: Reiter's syndrome 
Explanation: Ans. is 'a' i.e., Reiter's syndrome