This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Dermatophytosis, Subject name: Skin
Onychomycosis is most commonly caused by - 
Select and explain. 
A. Trichophyton rubrum, B. Trichophyton mentagrophytes, C. Epidermatophyton floccosum, D. Candida
A: Trichophyton rubrum
Answer: A: Trichophyton rubrum 
Explanation: Ans. is 'a' i.e., Trichophyton rubrum Tinea Unguium (Onychomycosis)o Tinea unguium is dermatophvtic infection of fingers and toe nails. Most common causative species is T-rubrum. Toe nails are more commonly involved. T. Unguium may be of two types : -Distal subungal onychomycosis : - This is most common type offungal infection of nail (90%). It starts at the distal edge of the nail plate and slowly growrs inwards to involve entire nail plate.Proximal Subungal onychomycosis : - Starts at the base of nail and slowly involves the entire nail plate.