Athlete's foot is caused by a variety of different fungal infections. The presentation of a severe case would look much like the photograph here. The interdigital spaces and any moist areas of the foot are likely to be affected first. As the disease progresses, the nail and nail bed become affected. Once the nails are infected, the course of treatment must involve three different areas. First, the feet must be kept clean and dry. This includes allowing shoes to air dry for 24 hours between wearings, wearing shoes made of leather or other "breathable" materials, using antifungal powders inside the shoes, changing socks at least twice daily, wearing sandals whenever practical, washing the feet with soap twice daily, and applying an astringent. Additional precautions include always wearing shower shoes in public showers/locker rooms and not sharing towels or wearing another person's shoes to prevent contact with the fungus and reinfection. Second, an appropriate antifungal cream, such as clotrimazole, should be applied to clean, dry feet daily. This helps to kill the infection from the outside toward the inside of the body. Third, in advanced cases that involve the nail bed, oral medications are necessary for at least 12 weeks and often up to 6 months (until the new nail is visible and free of fungal discoloration and texture). This kills the fungus from the inside of the skin toward the surface.
What would be the most effective oral agent to use for treating the condition depicted in this case?