This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Skin
Which of the following is the most common side effect of Isotretinoin used for acne vulgaris? 
Select and explain. 
A. Facial erythema, B. Xerosis, C. Hair loss, D. Cheilitis
D: Cheilitis
Answer: D: Cheilitis 
Explanation: Cheilitis is the most common side effect of isotretinoin. Ref: Rook Textbook of Dermatology, 8th edition, Page 42.64