Answer
The adrenal gland.
Work Step by Step
In a normal adrenal gland, the low dose of dexamethasone will cause blood cortisol levels to drop as ACTH is inhibited. In an overactive adrenal gland, it continues to produce high levels of cortisol despite the inhibition of ACTH. It’s worth noting that this is not an extremely precise test and the rate of false negatives is high as the adrenal gland responds despite malfunctioning at times.
