An otherwise well 43 year old man visited Mexico.He was well until he returned to the United States a few days later,whereupon he experienced bloody diarrhoea and crampy abdominal pain.Because of the possibility of parasitic infection,a colonic biopsy was performed.A representative section from the procedure is shown in the accompanying photomicrograph.Based upon the histopathologic findings,which one of the following is the best diagnosis?

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The high power photomicrograph shows luminal Entamoeba histolytica amoebas (labelled "A"),which appear as large cells,often containing red blood cells (RBCs),with a very pale nucleus when compared to human cells in the colonic mucosa (labelled "M").The most common form of infection is amoebic dysentery,where the Entamoeba invades the mucosa of the colon and rectum,causing painful,bloody diarrhoeAPathologically,the mucosa is extensively undermined,producing typical flask-shaped ulcers,which may perforatEEntamoeba may also spread to other organs in the body,especially the liver,lungs,and brain,but any organ can be involveD

An Otherwise Well 43 Year Old Man Visited Mexico