Explanation:

When a patient on TMP-SMX presents with a petechial rash, it can be indicative of a severe adverse reaction such as thrombocytopenia or a hypersensitivity reaction. The appropriate action for the nurse to take in this situation is to hold the dose of TMP-SMX and notify the healthcare provider immediately. This is crucial to prevent further administration of a medication that may be causing a serious adverse effect. Requesting a blood glucose level (Choice B) is not relevant in this scenario as the patient's presentation is suggestive of a skin-related issue rather than a glucose-related problem. Similarly, requesting a BUN and creatinine level (Choice C) would not address the immediate concern of a petechial rash and its association with TMP-SMX. Requesting an order for diphenhydramine (Choice D) may help manage itching or mild allergic reactions but is not the priority when a petechial rash is observed, as it may indicate a more severe reaction requiring immediate intervention.