<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 19-year-old female student presents to her physician for overall fatigue. She is having a hard time concentrating while studying and feeling tired most of the time. She also has had constipation for more than 3 weeks and rectal bleeding on occasions. She notices she is getting colder and often needs to wear warmer clothes than usual for the same weather. On examination, a small nodule around the size of 1cm is palpated in the left thyroid lobule; the gland is nontender. There is no lymphadenopathy. Her vital signs are: blood pressure is 11875 mm Hg, respirations are 17min, pulse is 87min, and temperature is 36.1C 97.0F. Which of the following is the best next step in the management of this patient?? A: Serum T3 levels, B: Thyroid ultrasound, C: Serum calcitonin levels, D: Radionuclide thyroid scan, E: Combination T4 and T3 therapy,  <response>: This is a clinical vignette describing a 19-year-old female with symptoms of fatigue, constipation, cold intolerance, and a thyroid nodule on exam. The stem is asking for the best next step in management. Based on the clinical presentation, the differential diagnosis includes hypothyroidism and thyroid cancer. Hypothyroidism can cause fatigue, constipation, cold intolerance, and thyroid enlargement. Thyroid cancer, especially medullary thyroid cancer, can present with a thyroid nodule. The correct next step would be to confirm the diagnosis with appropriate lab testing and imaging. Serum T3 levels would not be useful, as T3 is often normal in hypothyroidism. Serum calcitonin screens for medullary thyroid cancer but would be premature without imaging. Radionuclide scanning helps characterize thyroid nodules but ultrasound should come first. Combination T4T3 therapy would be treatment, not diagnosis. Thyroid ultrasound is the best choice to visualize the nodule and determine if fine needle aspiration is needed.B: Thyroid ultrasound 