Fecal occult blood
Fecal occult blood ( FOB ) refers to blood in the feces that is not visibly apparent ( unlike other types of blood in stool such as melena or hematochezia ) . 
<triplet> melena <subj> blood in stool <obj> subclass of <subj> hematochezia <obj> different from <triplet> hematochezia <subj> melena <obj> different from