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
[{'content': "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? \nSelect and explain. \nA. Wood's lamp examination, B. KOH mount, C. Slit smear (split skin smear), D. Gram stain", 'role': 'user'}, {'content': 'Answer: B \nExplanation: Discolouring and tunneling of the fingernails suggests onychomycosis (Tinea unguium)\nOnychomycosis refers to nail infections caused by any fungus, including yeasts and nondermatophyte molds.\nKOH mount is used for fungal infections.', 'role': 'assistant'}]