Which part of the nail forms the seal between the free edge of the nail and the skin of the fingertip?

The correct answer is the hyponychium, which is located underneath the free edge of the nail. The primary role of the hyponychium is to form a seal that protects the area beneath the nail from external elements, such as bacteria and dirt. This sealing function is vital for maintaining nail health and preventing infections, as it helps to keep the nail and fingertip area secure.

In contrast, the cuticle refers to the thin layer of dead skin that overlaps the nail plate at the base of the nail; it serves to protect the area where the nail grows from pathogens but does not form a seal at the fingertip. The nail bed is the skin beneath the nail plate, providing support and nourishment to the nail as it grows. The nail matrix is the tissue located at the base of the nail that generates new nail cells and determines the shape and thickness of the nail, but it is not involved in sealing the tip of the finger.