The formation of a carious lesion in a previously sound tooth follows a set sequence. Which one of the following events describes the final stage of this sequence?

The final stage in the sequence of carious lesion formation occurs when caries enters the dentine layer. This milestone indicates that the decay has progressed past the enamel, which is the outermost protective layer of the tooth, and has breached into the underlying dentine, a softer tissue that is more susceptible to carious infection.

At the initial stage, demineralisation of the enamel might occur, which can lead to visual signs of decay, but once the carious process advances beyond this stage and penetrates into the dentine, it signifies a more severe level of tooth decay. The dentine layer is not only softer but also contains nerve endings, which may lead to increased sensitivity or pain for the patient. This progression can lead to further complications, such as pulpitis, if left untreated, highlighting the importance of early detection and intervention at the initial stages of caries development.