Final answer:
The thyroxine injections would result in an increased BMR in the hypophysectomized rat. This is because thyroxine acts to raise the BMR which is typically low when the pituitary gland, which stimulates thyroid hormone production, is absent. The correct option is A.
Step-by-step explanation:
The effect of thyroxine injections on the hypophysectomized rat's basal metabolic rate (BMR) would be A. Increased BMR.
Thyroxine is a thyroid hormone that regulates metabolic activity. In hypophysectomized rats, the pituitary gland is removed, which can lead to a decrease in thyroid stimulating hormone (TSH) and, as a result, a decreased production of thyroxine.
By injecting thyroxine, you replace the missing hormone, thus increasing the BMR back towards normal levels. This is consistent with what occurs in hypothyroidism, where BMR is low and administering thyroid hormones is necessary to raise the BMR.
Patients with hyperthyroidism typically have a high BMR due to overproduction of thyroid hormones such as thyroxine (T4) and triiodothyronine (T3), leading to symptoms like weight loss and increased heart rate. Conversely, patients with hypothyroidism present with a low BMR, often resulting in weight gain and cold sensitivity. The correct option is A.