A 27-year-old woman with anxiety and weight loss presents with tachycardia and tremors. What mechanism accounts for her weight loss?

The correct answer is based on the physiological mechanisms associated with an increased metabolic rate. In this case, the patient is presenting symptoms that suggest hyperthyroidism, such as anxiety, weight loss, tachycardia, and tremors. Hyperthyroidism is characterized by an excess of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), which increases the body's metabolic rate.

When the metabolic rate is elevated, the body utilizes glucose and fat stores more rapidly for energy, leading to weight loss even if the individual may be eating normally. This heightened metabolism can cause the catabolic processes to outpace anabolic processes, resulting in a negative energy balance.

The other options do not accurately describe the mechanism for the weight loss observed in hyperthyroidism. A deficiency in thyroid hormone would typically lead to weight gain due to a reduced metabolic rate, calcitonin does not play a significant role in metabolism and primarily aids in calcium regulation, and insulin resistance is more associated with weight gain, not weight loss, due to its impact on glucose metabolism and fat storage. Therefore, the increased metabolic rate is the mechanism responsible for this patient’s weight loss.