Final answer:
The priority nursing assessment for the client is A. Airway patency, due to the symptoms indicating a potential allergic reaction which can compromise the airway.
Step-by-step explanation:
The priority nursing assessment for a client who presents with a rash, difficulty breathing, chest tightness, weakness, a rapid and thready pulse, and low blood pressure is A. Airway patency. The symptoms reported, particularly difficulty breathing and chest tightness, suggest that the client might be experiencing an allergic reaction, possibly to the sulfasalazine they have been taking.
Ensuring the airway is clear is vital because airway compromise can quickly lead to respiratory failure and is a life-threatening condition. Other assessments, such as checking for peripheral edema (Option B), urine discoloration (Option C), and gastrointestinal disturbances (Option D), are important but secondary to establishing and maintaining a patent airway.