mrcem2-1808

You assess a patient who came in with chest discomfort and difficulty breathing. They have been diagnosed with a spontaneous pneumothorax and their initial attempt at pleural aspiration was unsuccessful. The pneumothorax is still significant in size, and the patient continues to experience breathlessness. You get ready to insert a Seldinger chest drain into the ‘safe triangle’.
What is the lower boundary of the ‘safe triangle’?