Final answer:
Sealants are contraindicated for teeth with proximal decay because the presence of decay indicates active dental disease, which the sealant would cover, potentially allowing the decay to progress undetected.
Step-by-step explanation:
Contraindications to sealant placement includes proximal decay because the sealant is unable to adhere properly to surfaces that are not sound. Dental caries (cavities) are microbial lesions that damage the teeth, beginning with the outer enamel and potentially reaching the underlying dentin or the innermost pulp if left untreated. Sealants are designed to act as a barrier, protecting enamel from plaque and acids that contribute to tooth decay. However, if there is decay, especially in the areas between the teeth (proximal decay), it indicates active disease which a sealant could obscure and allow to progress undetected. This can lead to more severe cavities, possible tooth loss, and other complications such as abscesses, which can spread to the bloodstream if not properly managed.