A 22-year-old man is brought to the emergency department after a motorcycle accident. He was initially briefly unconscious, but then regained full consciousness and reported feeling fine. However, one hour later, he became confused and drowsy, and his right pupil is now dilated and nonreactive. A non-contrast CT scan of the head shows a biconvex (lentiform) hyperdense collection that does not cross suture lines. Which of the following is the most likely diagnosis?
