Diagnostic test that consists of visualizing the esophageal mucosa, stomach, and part of the duodenum, to see what is their status, to take samples or small resections if necessary. For this, a tube is inserted called as a gastroscope that contains a lens at the tip and which allows to analyze in real time the status of the different structures through which it passes. It does not require any specific preparation, except to be fasting for at least 6 hours before, nor sedation, though in some cases if administered to make it more tolerable.