**Core Concept:** Hemorrhagic colitis is a type of enterocolitis (inflammation of the colon) caused by the ingestion of contaminated food or water. Escherichia coli (E. coli) O157:H7 is a particularly harmful strain that can lead to hemorrhagic colitis. Diagnosis of this infection relies on isolating the bacteria from the patient's stool sample.

**Why the Correct Answer is Right:** To diagnose E. coli O157:H7 infection, the lab staff needs to isolate the bacteria from the patient's stool sample. MacConkey agar is a selective medium that helps differentiate lactose-fermenting bacteria (like E. coli O157:H7) from other bacteria by the production of lactose fermentation. The color change from pink to yellow or orange indicates the presence of lactose fermenting bacteria, which in this case is indicative of E. coli O157:H7.

**Why Each Wrong Option is Incorrect:**

A. Blood agar is a general culture medium that supports the growth of various bacteria. While it might grow some E. coli, it does not specifically identify E. coli O157:H7.

B. Xylose lysine desoxycholate (XLD) agar is a selective medium for Salmonella and Shigella, not E. coli O157:H7.

C. Brilliance® E. coli agar is another selective medium for E. coli, but it is not specifically designed for E. coli O157:H7. MacConkey agar is more effective in isolating E. coli O157:H7 due to its capacity to differentiate lactose-fermenting bacteria.

D. Blood agar is a general culture medium that supports the growth of various bacteria, but it does not specifically identify E. coli O157:H7.

**Clinical Pearl:** MacConkey agar is the most suitable selective medium to isolate and identify E. coli O157:H7 due to its ability to differentiate lactose-fermenting bacteria. This helps in the accurate identification of the pathogen causing the illness in this case, which is crucial for appropriate antibiotic therapy and patient management.