mrcem2-0232

A 35-year-old female smoker presents with an episode of acute right-sided chest pain. She describes the pain as being ‘extremely severe’, and it is aggravated by movement and coughing. The pain radiates to her neck and shoulder on the affected side. Her vital signs are within normal limits, and her oxygen saturation is 98% on room air. On examination, she exhibits localized tenderness around the 4th rib on the right-hand side. Her lung fields are clear, and her heart sounds are normal.
What is the SINGLE most likely diagnosis?