Final answer:
The client should discontinue Ibuprofen when starting a new prescription for lithium due to the increased risk of lithium toxicity as NSAIDs can elevate lithium levels in the blood.
Step-by-step explanation:
When a client has a new prescription for lithium, it is important for the nurse to review the client's medication history for contraindications with over-the-counter (OTC) drugs. Among the options provided, the medication that the client should discontinue when taking lithium is Ibuprofen (A). Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can increase lithium levels in the blood, leading to an increased risk of lithium toxicity. Conversely, medications such as diphenhydramine, acetaminophen, and famotidine do not have the same degree of interactions with lithium and do not typically necessitate discontinuation unless directed by a healthcare provider.