Final answer:
The best first test for diagnosing onychomycosis is Potassium Hydroxide (KOH) preparation, which involves preparing a wet mount of the nail specimen using a potassium hydroxide solution. This test allows for the visualization of hyphae and fungal spores, which are indicative of a fungal infection.
Step-by-step explanation:
The best first test for diagnosing onychomycosis, a fungal infection of the nails, is Potassium Hydroxide (KOH) preparation. This test involves preparing a wet mount of the nail specimen using a potassium hydroxide solution. This solution dissolves the keratin in the nail, allowing the visualization of hyphae and fungal spores, which are indicative of a fungal infection.