Final answer:
Trismus is a distinguishing symptom of peritonsillar abscess (PTA) making it different from epiglottitis, which usually does not present with trismus but with symptoms like difficult breathing and hoarse voice.
Step-by-step explanation:
Trismus, which is characterized by the inability to fully open the mouth, occurs in over two-thirds of patients with peritonsillar abscess (PTA), making it a distinguishing symptom when compared to other conditions. The correct answer differentiating it from pharyngitis, tonsillitis, influenza, and epiglottitis is epiglottitis. This is because although trismus is commonly associated with PTA, epiglottitis is typically characterized by difficulty in breathing, a sore throat, and a hoarse voice but not trismus. It is important to differentiate between these conditions, as they have different treatments and levels of urgency with PTA potentially requiring surgical intervention or drainage.