**Core Concept**
EC O157:H7 is a Shiga toxin-producing strain of E. coli that can cause severe foodborne illness, often with bloody diarrhea. To diagnose this strain, a specific characteristic is exploited to differentiate it from other E. coli strains. This involves the use of a medium that takes advantage of the unique metabolic properties of EC O157:H7.

**Why the Correct Answer is Right**
EC O157:H7 is unable to ferment sorbitol, a sugar found in many media. This is a key characteristic that distinguishes it from other E. coli strains, which can ferment sorbitol. The inability to ferment sorbitol is due to the lack of sorbitol dehydrogenase enzyme in EC O157:H7. This enzyme is necessary for the breakdown of sorbitol. As a result, colonies of EC O157:H7 appear colorless or pale pink on sorbitol MacConkey agar, whereas other E. coli strains appear pink due to the production of acid from sorbitol fermentation.

**Why Each Wrong Option is Incorrect**
**Option A:** Hektoen agar is a selective and differential medium used for the isolation of enteric pathogens, but it is not specific for EC O157:H7. It contains bile salts and crystal violet to inhibit the growth of Gram-positive bacteria and to stain Gram-negative bacteria, respectively.
**Option C:** Blood agar is a general-purpose medium used for the isolation and growth of a wide range of bacteria. While it may support the growth of EC O157:H7, it does not provide a specific characteristic to distinguish it from other E. coli strains.
**Option D:** LJ (Lowenstein-Jensen) medium is used for the isolation and growth of Mycobacterium species, particularly M. tuberculosis. It is not relevant to the identification of EC O157:H7.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that EC O157:H7 is a Shiga toxin-producing strain of E. coli that can cause severe foodborne illness, often with bloody diarrhea. This strain is often associated with contaminated food and water, and prompt recognition is crucial for effective treatment and prevention of complications.

**✓ Correct Answer: B. Sorbitol**