A 5 monts baby present with recurrent vomiting. Mother noticed some of the vomitus is blood stained. Choose the single most likely investigation?

The key is A. Upper GI endoscopy. [Probable diagnosis is infantile pyloric stenosis. Pyloric stenosis usually affects babies between 2 and 8 weeks of age, but can occur anytime from birth to 6 months. Slight hematemesis of either bright-red flecks or a coffee-ground appearance is sometimes observed. Preferred investigation is ultrasonography. If US can not establish the diagnosis we go for barium meal and if both are non-diagnostic we go for endoscopy. As the question wants “SINGLE” most likely investigation it means the definitive or gold standard investigation which is upper GI endoscopy].