Final answer:
The correct statement that shows an understanding of lithium toxicity is vomiting, which is an important sign to monitor and report for anyone taking lithium.
Step-by-step explanation:
The statement by the client that indicates an understanding of the manifestations of lithium toxicity is "Vomiting is an indication of toxicity." Lithium toxicity symptoms include gastrointestinal disturbances such as nausea, vomiting, and diarrhea. Early toxicity signs may resemble those of the flu, including symptoms such as loss of appetite, vomiting, weakness, and coordination issues. More severe toxicity can lead to confusion, abnormal heart rhythms, and seizures. Therefore, reporting vomiting is appropriate as it may indicate a need for immediate medical evaluation.