Unilateral posterior pharyngeal wall swelling is due to

Acute Retropharyngeal Abscess : The retropharyngeal space is divided longitudinally by a midline fibrous Raphe into two paramedian spaces known as space of Gillette which contains lymph nodes known as nodes of Rouviere. Acute suppurative lymphadenitis of these lymph nodes leads to an acute retropharyngeal abscess. On examination A unilateral para - a median bulge of the posterior pharyngeal wall is seen. (Ref: Scott Brown 8th edition, VOL 3 )