What is the first step in assessing new onset diarrhea for a patient who uses dietary fiber?

The correct choice involves performing a digital rectal exam as the first step in assessing new onset diarrhea for a patient who uses dietary fiber. This examination provides critical information about the rectal area and can help identify any potential abnormalities or causes of diarrhea such as inflammation, infection, or even impaction. Given that the patient has dietary fiber in their regimen, which can sometimes lead to changes in bowel habits, the digital rectal exam offers a direct and immediate method to assess for physical issues, offering insight into whether further investigation or another intervention is needed.

The other options would not be appropriate initial steps for assessing diarrhea. Conducting a full blood panel is more invasive and time-consuming and would typically be reserved for when there are more concerning signs or symptoms. Initiating dietary restrictions may not be necessary at the onset of new diarrhea, as it's important to determine whether the fiber intake is the underlying issue or if there is another cause. Requesting a colonoscopy is also more invasive and would generally be considered later in the diagnostic process, particularly if there are symptoms suggesting significant underlying pathology. Therefore, starting with the digital rectal exam allows for a focused and effective assessment approach.