<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 comes to the physician because of a 6-month history of muscle stiffness, myalgia, and a 7-kg 15-lb weight gain. Her last menstrual period was 4 months ago. Physical examination shows cold, dry skin, and proximal muscle weakness. Deep tendon reflexes are 2 bilaterally, with delayed relaxation. The creatine kinase level is 2,940 UL. Which of the following is the most appropriate next step in diagnosis?? A: Thyroid function tests, B: Serum assay for muscle specific tyrosine kinase antibody, C: Muscle biopsy, D: Serum electrolytes, E: Repetitive nerve stimulation,  <response>: This question describes a 48-year-old woman with symptoms of muscle stiffness, myalgia, weight gain, and menstrual irregularity. She has physical exam findings consistent with hypothyroidism, including cold dry skin, proximal muscle weakness, and delayed deep tendon reflex relaxation. The elevated creatine kinase indicates muscle damage. The symptoms, exam findings, and lab results point towards hypothyroid myopathy as the most likely diagnosis. Thyroid function tests would be the appropriate next step to confirm the diagnosis of hypothyroidism. Muscle biopsy and nerve conduction studies may be useful to evaluate the myopathy but are not needed to establish the etiology. Serum electrolytes would not be helpful. Muscle tyrosine kinase antibody testing would evaluate myasthenia gravis which does not fit with the clinical presentation.A: Thyroid function tests 