Which of the following structures is invaded by the genus Trichophyton?

The genus Trichophyton is known for causing various dermatophytic infections, and it can indeed invade multiple keratinized tissues in the human body. Trichophyton species can invade hair, contributing to conditions such as tinea capitis (scalp ringworm), where the fungi infect the hair shaft and follicles. They also commonly infect nails, leading to onychomycosis, where the fungus penetrates the nail bed, causing discoloration and thickening of the nails. Furthermore, Trichophyton can infect the skin, resulting in conditions like tinea corporis (ringworm of the body) and tinea pedis (athlete's foot), which affect the outer skin layer.

Since Trichophyton can efficiently invade all these structures—hair, nails, and skin—the most comprehensive and accurate answer is that it invades all of these options. This highlights the versatility and pathogenic potential of the genus in causing superficial fungal infections.