Which of the following medications is used for ADHD under the brand name Intuniv ER?

Guanfacine is indeed the medication marketed under the brand name Intuniv ER for the treatment of Attention-Deficit/Hyperactivity Disorder (ADHD). This medication belongs to the class of alpha-2 adrenergic agonists and is specifically used to help manage symptoms of ADHD in both children and adolescents. Intuniv ER provides a once-daily dosing option which can improve adherence and ensure consistent symptom control throughout the day.

While clonidine and guanfacine are related as both can be used to treat ADHD, Intuniv is specifically the extended-release form of guanfacine. Clonidine, on the other hand, is another medication that can be used for ADHD under different brand names, but not Intuniv ER. Methyldopa is primarily used to manage hypertension and is not indicated for ADHD treatment, while Duraclon is the name for an injectable form of clonidine, which also does not relate directly to the extended-release formulation of guanfacine. Therefore, the association of Intuniv ER with guanfacine makes it the correct answer.