What nail disorder is characterized by white spots found inside or under the nail plate?

Leukonychia is the correct choice as it specifically refers to the white spots or streaks that appear on the nail plate. These white marks can occur due to trauma, infection, or other factors affecting the nail's growth. The term "leukonychia" itself is derived from Greek, where "leuko" means white and "onych" refers to the nail. This disorder is often benign and resolves on its own as the nail grows out.

In contrast, the other terms refer to different nail conditions. Acanthosis generally relates to skin changes rather than nails. Onychomycosis is a fungal infection of the nail, usually characterized by discoloration and thickening rather than isolated white spots. Pterygium is a condition that involves the attachment of the skin to the nail bed, not white spots on the nail itself. Thus, leukonychia distinctly identifies the condition of having white spots within the nail plate.