A weakened immune system may be caused by immunodeficiency disorders.

Explanation: Immunodeficiency disorders are conditions that result in an impaired or weakened immune response, making it more difficult for the body to fight off infections and diseases. This can happen due to genetic factors, autoimmune diseases, infections like HIV, or certain medications that suppress the immune system. The other options mentioned (lysozymes, muscle disorders, and complement systems) do not primarily lead to a weakened immune system.