Which is indicated by a positive urea breath test?

The patient is brought to the emergency department complaining of sudden onset crushing substernal chest pain. He is pale, diaphoretic, and can only speak in short sentences. His vital signs are notable for a heart rate of 118 bpm, respiratory rate of 22, and a blood pressure of 94/52 mmHg. Upon assessment, peripheral pulses are palpable but weak; lungs are clear; mild jugular venous distention is noted. The emergency medical services administered aspirin on arrival to the scene. The nurse anticipates performing which intervention first? 

The nurse is taking care of a patient with symptoms concerning for Cushing’s syndrome/disease, namely moon facies, acanthosis nigricans, hair loss, thinning of skin, weight gain, hyperglycemia, hypertension, and osteoporosis. Initial diagnostic tests found the patient’s Cushing syndrome to be ACTH-dependent. Which of the following diagnostic tests would best differentiate between an ectopic ACTH-producing source versus a pituitary source?