The clinical diagnosis of pulpitis—whether reversible or irreversible—relies heavily on the patient’s symptoms and diagnostic tests. Among the options provided, prolonged spontaneous pain is the most reliable indicator of irreversible pulpitis.

- Sharp pain on biting often suggests cracked tooth syndrome or occlusal trauma rather than pulp inflammation per se. It is not specific for irreversible pulpitis.

- Prolonged spontaneous pain refers to pain that occurs without any stimulus and lingers for an extended period (minutes to hours). This symptom reflects inflammation extending beyond the threshold where the pulp can heal, indicating irreversible pulpitis.

- Sensitivity to cold that resolves quickly is typical of reversible pulpitis, where the pulp is inflamed but still capable of healing once the irritant is removed.

- Mild discomfort only on thermal stimulation also suggests reversible pulpitis or a healthy pulp with minor irritation, not irreversible damage.

Key takeaway:
- Irreversible pulpitis is characterized by spontaneous, lingering pain.
- The presence of prolonged spontaneous pain signals that the pulp’s inflammation is severe and cannot recover, necessitating endodontic treatment (root canal therapy).

Reference: Cohen’s Pathways of the Pulp, 12th Edition, Chapter 5: Pulpal and Periapical Diagnosis / Pages 127-130