Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) moyamoya disease ?
    ### Response:
    Moyamoya disease is a disorder of blood vessels in the brain, specifically the internal carotid arteries and the arteries that branch from them. These vessels, which provide oxygen-rich blood to the brain, narrow over time. Narrowing of these vessels reduces blood flow in the brain.
    