This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Psoriasis, Subject name: Dental
[{'content': 'The commonest sign of nail psoriasis is: \nSelect and explain. \nA. Pitting, B. Pterygium, C. Oil drop sign, D. Onycholysis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Nail changes in psoriasis - Most common - Nail pitting/Thimble pitting Most characteristic - oil drop sign/ salmon patch Onycholysis Leukonychia Subungual hyperkeratosis Nail tunnel', 'role': 'assistant'}]