In caring for a client who is receiving linezolid IV for nosocomial pneumonia, which assessment finding is most important for the nurse to report to the healthcare provider?

A. Watery diarrhea is a potential sign of Clostridium difficile (C. diff) infection, a serious and common complication associated with antibiotics like linezolid. B. Nausea and headache are side effects but are less urgent than symptoms suggesting C. diff. C. Increased fatigue is non-specific and may be related to the infection or the medication, but it is not as immediately concerning as diarrhea. D. Yellow-tinged sputum is typical of pneumonia and does not indicate an urgent issue related to linezolid therapy.