**Core Concept**
EHEC O157:H7 is a serotype of Shiga toxin-producing Escherichia coli (STEC), a bacterium that causes severe foodborne illness. The diagnosis of EHEC O157:H7 typically involves the isolation and identification of the organism from stool specimens using specific culture media.

**Why the Correct Answer is Right**
The correct answer is MacConkey agar with cefixime and tellurite supplement. This selective and differential medium is specifically designed to inhibit the growth of non-O157 E. coli while allowing the growth of EHEC O157:H7. The cefixime and tellurite supplement inhibit the growth of Gram-positive bacteria and Gram-negative bacteria that do not produce the O157 antigen, respectively. The selective pressure of cefixime and the inhibitory effect of tellurite allow for the isolation of EHEC O157:H7 from stool specimens.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood agar is a general-purpose culture medium that is not selective for EHEC O157:H7 and would not inhibit the growth of other bacteria.
**Option B:** EMB agar is a selective medium for the isolation of Gram-negative bacteria, but it is not specific for EHEC O157:H7.
**Option C:** Salmonella-Shigella agar is a selective medium for the isolation of Salmonella and Shigella species, but it is not specific for EHEC O157:H7.

**Clinical Pearl / High-Yield Fact**
EHEC O157:H7 produces Shiga toxins that can cause hemolytic uremic syndrome (HUS), a severe complication of E. coli infections. The use of specific culture media like MacConkey agar with cefixime and tellurite supplement is crucial for the early detection and isolation of EHEC O157:H7 from stool specimens.

**Correct Answer:** C. MacConkey agar with cefixime and tellurite supplement.