Final answer:
When monitoring a patient with hypomagnesemia, muscle weakness is a common symptom to look for, alongside other possible signs such as hypotension and disorientation.
Step-by-step explanation:
When caring for a client with hypomagnesemia due to long-term therapy with lansoprazole, it is important to monitor for several potential manifestations. Among the options provided, muscle weakness (c) is a common symptom of hypomagnesemia.
Since magnesium plays a crucial role in muscle and nerve function, its deficiency can lead to symptoms such as muscle cramps, tremors, and weakness. Additionally, the nurse should be aware of other signs of hypomagnesemia, such as hypotension (b), changes in mental status like disorientation (d), and arrhythmias, although bradycardia (a) is not typically directly linked to low magnesium levels.