Final answer:
The clinical diagnosis of acute apical periodontitis is most effectively supported by the symptom of marked tenderness of the tooth to touch and pressure, indicating inflammation of the periapical tissue.
Step-by-step explanation:
The clinical diagnosis of acute apical periodontitis is best supported by the finding of "Marked tenderness of the tooth to touch and pressure." This symptom is a classic indication of inflammation and irritation of the periapical tissue that occurs in acute apical periodontitis, a condition associated with periapical inflammation due to bacterial invasion of the pulp. The other symptoms described, such as a sensitive response to electric pulp test, pain that is difficult to localize, and sharp pain in response to cold, although indicative of dental issues, do not directly point to acute apical periodontitis as accurately as marked tenderness does.
It is important to understand that during the progression of periodontal disease, the inflammation can spread from the gums to the deeper periodontal structures, including the cementum and alveolar bone, as seen in periodontitis.