Which symptom would require immediate attention for a client taking an MAOI?

A headache in a client taking pseudoephedrine while on an MAOI is particularly concerning due to the potential for hypertensive crisis. Monoamine oxidase inhibitors (MAOIs) can interact with certain medications and foods, leading to dangerously elevated blood pressure. Pseudoephedrine is a sympathomimetic drug that can increase blood pressure and heart rate, especially when combined with MAOIs. Thus, a headache in this context may be indicative of a hypertensive crisis, which requires immediate medical attention.

In this scenario, the other symptoms listed do not carry the same level of urgency concerning MAOI interactions. Dry mouth related to buspirone, nausea from metformin, and dizziness when standing up may require monitoring and management but do not pose an immediate risk like the potential hypertensive crisis associated with using pseudoephedrine in conjunction with an MAOI.