<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 48-year-old woman presents to her primary care physician with the complaints of persistent fatigue, dizziness, and weight loss for the past 3 months. She has hypothyroidism for 15 years and takes thyroxine replacement. Her blood pressure is 9060 mm Hg in a supine position and 6540 mm Hg while sitting, temperature is 36.8C 98.2F and pulse is 75min. On physical examination, there is a mild increase in thyroid size, with a rubbery consistency. Her skin shows diffuse hyperpigmentation, more pronounced in the oral mucosa and palmar creases. The morning serum cortisol test is found to be 3 μgdL. Which of the following is the best next step in this case?? A: Serum adrenocorticotropic hormone ACTH, B: Plasma aldosterone, C: Adrenocorticotropic hormone ACTH stimulation test, D: Adrenal imaging, E: 21-hydroxylase antibodies,  <response>: This is a patient with long-standing hypothyroidism on thyroxine replacement who now presents with hypotension, fatigue, dizziness and weight loss. The key finding is a low morning serum cortisol level of 3 μgdL, which indicates adrenal insufficiency. The differential diagnosis includes primary adrenal insufficiency e.g. autoimmune adrenalitis, adrenal hemorrhageinfarction, infections, metastases and secondary adrenal insufficiency pituitary disease causing ACTH deficiency. To distinguish between primary and secondary adrenal insufficiency, the next best step is to check plasma ACTH level. In primary adrenal insufficiency, ACTH will be elevated due to loss of negative feedback from low cortisol. In secondary adrenal insufficiency, ACTH will be low or inappropriately normal due to pituitary dysfunction.Choices A and C ACTH level and stimulation test are relevant, while choices B, D and E are not useful next steps.C: Adrenocorticotropic hormone ACTH stimulation test 