Final answer:
The 16-year-old girl, as the legal guardian of her 6-month-old son, must give informed and voluntary consent for her son's lumbar puncture.
Step-by-step explanation:
To legally obtain informed consent for a lumbar puncture on a 6-month-old son, it must be ensured that the patient giving consent, in this case, the mother who is a 16-year-old girl, is the legal guardian of the infant. While the mother is a minor, if she is the legal guardian of the child, she has the authority to consent to medical procedures on behalf of her child. This is because parents and legal guardians can provide consent for those under their care. The informed consent process entails that the guardian must be fully informed about the procedure, including its risks and benefits, and must give voluntary consent without any coercion.