The most common route for microorganisms to reach the dental pulp is through carious lesions and cavity preparations. This is because dental caries cause enamel and dentin breakdown, allowing bacteria to directly invade the pulp chamber. Cavity preparations also create a direct communication between the oral environment and the pulp. Other routes, like periodontal ligament or blood supply, are less direct and less frequent pathways for microbial invasion. Therefore, maintaining proper caries control and aseptic techniques during cavity preparation is critical to prevent pulpal infections.

Reference: Pathways of the Pulp, Cohen & Hargreaves, 12th Edition.