Final answer:
The nurse should monitor the client with hypomagnesemia for signs of hyperactivity and hypokalemia, which are associated with low magnesium levels.
Step-by-step explanation:
A nurse who is caring for a client with hypomagnesemia due to long-term therapy with lansoprazole should monitor for various manifestations including changes in neuromuscular and cardiovascular function. Hypomagnesemia can lead to overexcitability of the nervous system, similar to hypocalcemia. Considering the options given, hyporeflexia would not be expected, as low magnesium typically causes increased neuromuscular excitability, resulting in signs such as tremors, muscle cramps, and hyperactivity. Hypertension is not directly related to hypomagnesemia. Hypokalemia is another possible complication since hypomagnesemia can cause a decrease in potassium levels, leading to muscle weakness, cardiac arrhythmias, and other symptoms. Therefore, among the options provided, the nurse should particularly monitor for hyperactivity and hypokalemia due to their close association with hypomagnesemia.