A 36-year-old woman presents to the emergency department with left wrist pain and swelling that began immediately after a fall earlier today. She was walking her dog when the leash was abruptly pulled, causing her to fall forward onto outstretched hands. She reports pain in the left wrist that worsens with pinching and grasping. Physical examination shows moderate swelling and mild tenderness over the left wrist. Her left thumb is currently immobilized in a thumb spica splint. A wrist radiograph obtained 2 days later is shown. Which of the following bones is most likely fractured in this patient?