Which symptom should prompt immediate assessment in a client taking antihypertensive medication?

The symptom that should prompt immediate assessment in a client taking antihypertensive medication is a severe headache. This is significant because a severe headache may indicate a potential hypertensive crisis or adverse reaction to the medication, especially in patients who are being treated for high blood pressure. This type of headache could also suggest that the blood pressure may be excessively low if the medication has reduced it too much, leading to insufficient perfusion and resulting in symptoms like a headache.

In contrast, while symptoms such as swelling of the ankles and increased fatigue can indicate side effects or complications related to antihypertensive therapy, they are generally not as urgent as a severe headache. Swelling of the ankles may suggest fluid retention or heart failure, which, while concerning, demands monitoring rather than immediate assessment. Increased fatigue could also be a side effect of antihypertensive medications, but it is typically a more gradual and less alarming symptom. Chest pain is indeed serious and warrants immediate attention, but when considering the context of antihypertensive treatment, a severe headache stands out as a critical issue that directly relates to potential blood pressure mismanagement.