Response needed for following post from peer: While instructing an APN student y

Response needed for following post from peer:
While instructing an APN student you are told she is seeing a 19 yo female with cellulitis of the
right leg that occurs 24 hours after shaving her legs. 
Why is it important to differentiate the potential cause before initiating empiric therapy? 
It is important to differentiate the potential cause of the cellulitis because we need to treat this patient effectively, therefore, it would be best to identify the exact cause so the correct antibiotic can be prescribed. 
What would you consider for empiric therapy why?
For empiric therapy, I would be sure to prescribe a broad-spectrum antibiotics that covers all gram positive bacteria. For example, I would prescribe cephalexin first to start this patient on the proper treatment. 
What are the monitoring parameters of efficacy and side effects of the agent you picked?
The monitoring parameters of efficacy would be to measure and watch the patients leg. When the patient first arrives with cellulitis, markings should have been made on the leg and measurements taken. Other monitoring parameters will be seen at through the patients blood levels. For example, her white blood cell count is very important to watch. Therefore, keeping a close eye on this area is extremely important to be sure her medications are working. Some side effects will include: pruritus, urticaria, edema, a maculopapular rash or even edema of the airway. 
After being treated for cellulitis she developed severe diarrhea, how will you determine if this is simple antibiotic associated diarrhea or a Clostridium Difficile infection? If a C.Diff infection how will you treat it?
In order to best determine if her diarrhea is associated with a C. Diff infection or not, would be by collecting a stool sample to test for the C. Diff toxins. If C. Diff infection is found, I would treat the patient with Vancomycin PO.