Which of the following tooth tissues is not composed of calcium hydroxyapatite crystals?

The pulp is the correct answer because it is composed primarily of soft connective tissue, which includes blood vessels, nerves, and cells, rather than mineralized tissues. Unlike enamel, dentine, and cementum, which are all mineralized structures containing calcium hydroxyapatite crystals that contribute to their hardness and strength, the pulp does not have this mineral content. The pulp serves important functions, such as providing nourishment to the tooth and maintaining the vitality of the dental tissues, but it does not share the same mineral composition as the other tooth structures.

Enamel is the hardest tissue and is primarily made of hydroxyapatite, providing protection for the underlying dentine. Dentine, while not as hard as enamel, also contains hydroxyapatite in its structure. Cementum, which covers the root of the tooth, similarly consists of hydroxyapatite, helping anchor the tooth within the jawbone. Hence, the presence of hydroxyapatite is crucial for the structural integrity of these mineralized tissues, which the pulp lacks.