Final answer:
Lithium is the drug that might have led to the low serum sodium level in the client with bipolar disorder.
Step-by-step explanation:
The drug that might have led to this condition is Lithium. Sodium abnormalities, including hyponatremia (low serum sodium levels), are commonly associated with the use of lithium in the treatment of bipolar disorder. Lithium can cause the kidneys to retain fluids, leading to dilutional hyponatremia. In severe cases, lithium can result in a potentially life-threatening condition called lithium-induced nephrogenic diabetes insipidus, which can further contribute to low serum sodium levels.