What is the structure beneath the nail that contains nerves, lymph, and blood vessels called?

The structure beneath the nail that contains nerves, lymph, and blood vessels is called the nail matrix. The nail matrix is crucial because it is responsible for producing cells that become the nail plate, the hard part of the nail. This area is located at the base of the nail, hidden under the cuticle. The presence of blood vessels allows for the supply of nutrients, while nerves provide sensation to the nail area.

On the other hand, choices like the nail mantle refer to the area that surrounds and protects the base of the nail, and the nail bed is the skin underneath the nail plate that supports the nail. The nail groove is the indentation that guides the growth of the nail, but it does not contain the vital structures mentioned in the question. Thus, the nail matrix, being the source of nail growth and containing essential biological structures, is the correct answer.