What is the diagnosis for purplish lesions on the buccal mucosa present since birth?

The presence of purplish lesions on the buccal mucosa that have existed since birth suggests a vascular lesion, which aligns well with haemangioma. Haemangiomas are benign tumors made up of a mass of blood vessels, typically appearing as red or purplish marks on the skin or mucosal surfaces. They are commonly found in children and can be present at birth or develop shortly thereafter.

While other options like naevus (which typically refers to a mole or pigmented lesion), erythema (which denotes redness due to dilation of capillaries), and fibroma (a tumor made up of fibrous or connective tissue) have characteristics that differ from the described purplish lesions, these conditions either do not commonly present as purplish lesions or are not typically present at birth.

Therefore, haemangioma stands out as the most fitting diagnosis, particularly due to its characteristic appearance and the timing of its onset.