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: Dental
A 60–year old male presented with discolouration, thickening and tunneling of 2 fingernails and one toe nail. Which of the following will clinch the diagnosis at the earliest? 
Select and explain. 
A. Wood's lamp examination, B. KOH mount, C. Slit smear (split skin smear), D. Gram stain
B: KOH mount
Answer: B: KOH mount 
Explanation: Discolouring and tunneling of the fingernails suggests onychomycosis (Tinea unguium)
Onychomycosis refers to nail infections caused by any fungus, including yeasts and nondermatophyte molds.
KOH mount is used for fungal infections.