This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Oral Cavity, Subject name: ENT
A 30-year-old male presented with trismus, fever, swelling pushing the tonsils medially and spreading laterally posterior to the middle of sternocleidomastoid. He gives a history of extraction of third molar few days back for dental caries. The diagnosis is: 
Select and explain. 
A. Retropharyngeal abscess, B. Ludwig's angina, C. Submental abscess, D. Parapharyngeal abscess
D: Parapharyngeal abscess
Answer: D: Parapharyngeal abscess 
Explanation: Abscess in the parapharyngeal space pushes the tonsil medially and it also lies deep to sternocleidomastoid. Trismus is due to spasm of medial pterygoid muscle. Retropharyngeal abscess pushes the posterior pharyngeal wall anteriorly while Ludwig's angina and submental abscess cause swelling in submental area.