Hi, I did review your concern Infectious mononucleosis generally presents with complains of pharyngitis and not rhinitis and sinus infection. I would suspect that she has a concomitant viral or bacterial rhinosinusitis. I would advice you to give her steam inhalation with vicks vapourub and at the same time consult a ENT doctor to start her on antibiotics if the snot has a yellowish thick texture and looks like a bacterial form of sinusitis. Investigations to confirm should be CT scan of paranasal sinuses and culture senstivity of the snot fluid I hope this helps wish you and your daughter a very healthy life ahead.