Final answer:
An expected adverse effect of lithium, which might lead a patient with bipolar disorder to discontinue its use, is weight gain. This is associated with lithium-induced hypothyroidism, along with other symptoms like cold sensitivity and reduced mental activity.
Step-by-step explanation:
The question pertains to the adverse effects that might cause a client with bipolar disorder to stop taking lithium, a common medication used in the management of this condition. Lithium can cause a variety of side effects, and one of the notable adverse effects that might lead a patient to discontinue the medication is weight gain, which is commonly associated with hypothyroidism induced by lithium. This hypothyroidism can also present with symptoms such as cold sensitivity and reduced mental activity. Therefore, the expected adverse effect that may have caused the client to stop taking lithium is weight gain (A). It's important for the nurse to recognize this potential adverse effect and address it with proper management strategies, which can include monitoring thyroid function and, if necessary, adjusting the treatment plan.