Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) fumarase deficiency ?
    ### Response:
    Fumarase deficiency is a condition that primarily affects the nervous system, especially the brain. Affected infants may have an abnormally small head size (microcephaly), abnormal brain structure, severe developmental delay, weak muscle tone (hypotonia), and failure to gain weight and grow at the expected rate (failure to thrive).
    