A 39-year-old man has had headache, weakness, and a 5-kg weight gain over the past 3 months. He has experienced mental depression during the same period. On physical examination, his face is puffy. His temperature is 36.9°C and blood pressure is 160/75 mm Hg. He has cutaneous striae over the lower abdomen and ecchymoses scattered over the extremities. A radiograph of the spine shows a compressed fracture of T11. Laboratory findings show fasting plasma glucose level of 200 mg/dL, serum Na+ of 150 mmol/L, and serum K+ of 3.1 mmol/L. The plasma cortisol level is 38 μg/dL at 8:00 am and 37 μg/dL at 6:00 pm. Administration of low and high doses of dexamethasone fails to suppress the plasma cortisol level and excretion of urinary 17-hydroxycorticosteroids. The plasma corticotropin level is 0.8 pg/mL. Which of the following lesions is most likely to be present in this man?

Correct Answer: B

Rationale: The clinical and laboratory features of this case point to Cushing syndrome. The dexamethasone suppression test is used to localize the source of excess cortisol. When low-dose and high-dose dexamethasone trials fail to suppress cortisol secretion, a pituitary corticotropin-secreting adenoma as the source of excess glucocorticoids is unlikely. The choice is an ectopic source of corticotropin, such as a lung cancer, or a tumor of the adrenal cortex that is secreting glucocorticoids. The plasma corticotropin level distinguishes between these two possibilities. Corticotropin levels are high if there is an ectopic source, whereas glucocorticoid secretion from an adrenal neoplasm suppresses corticotropin production by the pituitary, leading to atrophy of the contralateral adrenal cortex.

Correct Answer: A

Rationale: Isometric contraction: muscle tenses without shortening, as load exceeds force.

Correct Answer: A

Rationale: SV rises with EDV (preload) due to stretch; others are unrelated or opposite.

Correct Answer: D

Rationale: Hemorrhage reduces hydrostatic pressure; others compensate for blood loss.

Correct Answer: A

Rationale: Helper T cells release cytokines to activate B cells for antibody production.