A young boy presented with lesion over his right buttock which had peripheral scaling and central scarring. The investigation of choice would be –
The boy is having lupus vulgaris because the lesions have central scarring.
Central Scarring is a feature of lupus vulgaris.
Diagnosis of lupus vulgaris is confirmed by Biopsy.

Remember 
Feature                                                                    

Central clearing                                        
Central Scarring                                     
Central Crusting                                      

Disease

Tinea corporis  
 Lupus Vulgaris 
 Leishmaniasis   

 Investigation

KOH Smear
Biopsy
LD body demonstration
Tzank Smear
KOH preparation
Biopsy
Saboraud's agar