Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Acute intermittent porphyria ?
    ### Response:
    Acute intermittent porphyria (AIP) is one of the liver (hepatic) porphyrias. AIP is caused by low levels of porphobilinogen deaminase (PBGD), an enzyme also often called hydroxymethylbilane synthase. The low levels of PBGD are generally not sufficient to cause symptoms; however, activating factors such as hormones, drugs, and dietary changes may trigger symptoms.
    