An 80 year old woman suffering from RA presents with severe epigastric pain and vomiting. She also complains of shoulder tip pain. What is the single most discriminatory investigation?

The key is F. Erect CXR. [Elderly patient with RA is usually on NSAIDs which may lead to perforated peptic ulcer which presents as severe epigastric pain, vomiting and shoulder tip pain. Diagnosis is made by seeing free gas shadow under the diaphragm on erect CXR].