Which condition occurs when the pulp of a tooth is inflamed and causes pain, typically in a lower molar?

Acute pulpitis is characterized by inflammation of the dental pulp, which can result from various factors, including caries (decay), trauma, or thermal irritation. This condition often leads to significant pain, particularly in lower molars, due to their anatomical structure and the proximity of the pulp to the tooth's nerves. In acute pulpitis, the inflammation can be either reversible or irreversible, with irreversible pulpitis typically resulting in persistent and severe pain that may radiate to the jaw or ear.

Pericoronitis refers to inflammation of the gum tissue surrounding a partially erupted tooth, often seen with wisdom teeth, and does not involve the pulp itself. Periodontitis is a more chronic condition that affects the supporting structures of the teeth, including the gums and bone, leading to mobility of teeth rather than direct pulp inflammation. Dentin hypersensitivity involves discomfort in response to stimuli, like hot or cold, but does not necessarily indicate inflammation of the pulp.

Thus, the defining characteristic of acute pulpitis is the inflammation of the tooth's pulp, directly causing the pain, particularly noted in lower molars due to their common vulnerability to decay and subsequent pulp involvement.