A nurse is teaching a client who has a new prescription for lithium to treat bipolar disorder. The nurse should instruct the client to ensure an adequate intake of which of the following dietary elements?

Choice A reason: Vitamin C intake is important for general health but has no direct influence on lithium metabolism or toxicity. Choice B reason: This is correct because sodium balance directly affects lithium levels. Low sodium intake or excessive sodium loss (e.g., sweating, dehydration) can lead to lithium retention and toxicity. Adequate sodium intake helps maintain stable lithium levels and prevents dangerous fluctuations. Choice C reason: Potassium is essential for cardiac and neuromuscular function but does not directly interact with lithium metabolism. It is not the critical dietary element emphasized in lithium therapy. Choice D reason: Vitamin K is important for clotting but has no relationship to lithium therapy or toxicity.