Complex sentence: The involvement of toxin A and most importantly toxin B are the key element that determines the disease cause by "C. difficile", and in clinical laboratory have identified these toxins in the patients stools based on antibody and cytotoxicity assays.
Split into simpler sentences: The involvement of toxin A and--most importantly--toxin B is the key element that determines the disease caused by "C. difficile". Clinical laboratories have identified these toxins in patients' stool based on antibody and cytotoxicity assays.