In this case, a medical history was identified for this non-illness based film after inquiry with the patient.
If this was a de-novo lesion a differential for a lucent rib lesion should be followed. In fact, a popular film for fellowship viva examinations.
Because:
1. one needs to spot it (bones important review area of CXR); tests observation
2. give a sensible description of the findings; test of candidates descriptive skills
3. give a sensible differential; several for a lucent rib lesion from benign to malignant
4. check the candidate's advice/management skills
'What would you do next'? Tests if practical advice is given to help the clinician.