MSFinals-5773

A 56-year-old man has just been admitted to the medical ward. Two days ago, he returned from a business trip and his history suggests he may have caught an atypical pneumonia. While examining the patient’s chest clinically, you try to determine whether the pneumonia is affecting one lobe in particular or is affecting the whole lung.
On the right side of the patient’s chest, which one of the following surface landmarks would be most likely to mark the boundary between the middle and lower lobes?