The nurse is admitting a male client who takes lithium carbonate twice a day. Which information should the nurse report to the healthcare provider immediately?

A. A five-pound weight gain could be significant but might not require immediate reporting unless it continues to increase rapidly. B. A depressed affect is concerning but may not warrant immediate reporting unless it signals a severe change in the client's mental state. C. Nausea and vomiting could indicate lithium toxicity, a serious side effect requiring immediate attention and evaluation by the healthcare provider. D. Short-term memory loss can be a side effect of lithium but might not pose an immediate threat unless severe or rapidly worsening.