This patient should be treated with supportive care and clinical observation. His clinical picture is consistent with a likely viral upper respiratory tract infection, which is causing a pseudorelapse of his multiple sclerosis (MS). MS pseudorelapses involve a worsening of baseline neurologic symptoms (or recurrence of previous symptoms) that occurs in the setting of physiologic stressors. In this patient, the viral syndrome and associated systemic inflammatory response are the underlying cause of the pseudorelapse. Supportive treatment of these symptoms and observation to ensure that the patient returns to baseline status is the most appropriate management to avoid unnecessary treatments or changes in medication. Because actual inflammatory relapses can sometimes be triggered by pseudorelapses (and similar causes), treatment with glucocorticoids can be considered if improvement does not occur within days of resolution of the causative condition.

The patient most likely has a viral upper respiratory tract infection, with no evidence of a urinary tract infection. Without evidence of bacterial infection, antibiotic therapy, such as administration of amoxicillin-clavulanate, is not indicated.

The increased postvoid residual urine volume suggests that this patient has moderate urinary retention. An increase in the oxybutynin dosage would cause relaxation of the muscles of the bladder wall and would likely worsen the patient's retention.

Although glucocorticoid therapy might be appropriate therapy for a true MS relapse, treatment with methylprednisolone in this patient with a likely pseudorelapse is inappropriate.