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: Skin
A patient presents with erythematous scaly lesions on extensor aspect of elbows and knee. The clinical diagnosis is got by : 
Select and explain. 
A. Auspitz sign, B. KOH smear, C. Tzanck smear, D. Skin biopsy
A: Auspitz sign
Answer: A: Auspitz sign 
Explanation: Ans. is 'a' ie Auspitz signErythematous scaly lesions on extensor aspect of elbow and knee favours the diagnosis of Psoriasis.The clinical diagnosis of psoriasis can be made by demonstrating Auspitz sign.It is a characteristic finding of psoriasis in which removal of scale leads to pinpoint bleeding.Tzanck testis used in the clinical diagnosis of PemphigusNote that Tzanck smear is used in the diagnosis of herpes virus infection.KOH smearis used for diagnosing fungal infections such as ringworm infections