Aaron, a 42-year-old single father, goes to his physician complaining of nausea and chronic fatigue. He reports having felt fatigued and listless for about half a year, but he had attributed this to stress. He has lost considerable weight and, strangely, his skin looks tanned, even though he spends long hours at work and rarely ventures outside. His doctor finds very low blood pressure and a rapid, weak pulse. Blood tests show that Aaron does not have anemia, but his plasma glucose, cortisol, and Na⁺ are low, and his plasma K⁺ is high. His doctor orders an ACTH stimulation test, in which Aaron's secretion of cortisol is measured after he is given a synthetic form of ACTH. (a) What would account for Aaron's low plasma Na⁺ and high plasma K⁺ ? (b) What is the reason for doing an ACTH stimulation test? (c) Which gland is primarily affected if ACTH does not cause a normal elevation of cortisol secretion? What is this abnormality called? (d) Which gland is primarily affected if ACTH does cause an elevation of cortisol secretion?