Okay, let's tackle this question. The scenario is a child with fever, lymphadenopathy, and a grey membrane on the left tonsil. The question is about the definitive diagnostic media for the organism causing this.

First, I need to figure out what condition this is. Grey membranes on the tonsils are classic for diphtheria, caused by Corynebacterium diphtheriae. The key here is the grey membrane, which is a hallmark. So the next step is to determine which media is used to culture C. diphtheriae.

I remember that C. diphtheriae is a Gram-positive, club-shaped rod. For culture, they use specific media. Let me recall: Loeffler's serum slope is used for preliminary growth, but the definitive identification might require something else. Oh right, tellurite media like Wilson and Blair's agar is used because it allows for characteristic black colonies due to tellurite reduction. That's a key point. So Wilson and Blair's agar would be the correct answer.

Now, the options aren't listed here, but the correct answer is likely Wilson and Blair's tellurite agar. Let me check why the other options are wrong. For example, if one of the options was blood agar, that's not specific enough. Another might be a selective media for other organisms like Vibrio or something else. Another could be a medium for anaerobes, which isn't the case here. Also, maybe MacConkey agar, which is for Gram-negative bacteria, so that's out.

The clinical pearl here is that diphtheria's grey membrane and tellurite media are linked. Students should remember the media for C. diphtheriae to avoid confusion with other pathogens. Also, the importance of using the right media for definitive diagnosis, as other media might not support growth or differentiate the organism properly.

**Core Concept**
The grey membrane on the tonsil is classic for **diphtheria**, caused by *Corynebacterium diphtheriae*. Definitive diagnosis requires culturing the organism on **tellurite-containing media**, which inhibits normal flora and promotes characteristic **black colonies** due to tellurite reduction.

**Why the Correct Answer is Right**
**Wilson and Blair’s tellurite agar** is the gold standard for isolating *Corynebacterium diphtheriae*. The tellurite in the medium selectively inhibits most bacteria while allowing *C. diphtheriae* to grow as black colonies due to reduction of potassium tellurite to metallic tellurium. This contrasts with other media that fail to differentiate or support growth of this pathogen.

**Why Each Wrong Option is Incorrect**
**Option A:** *Blood agar* supports growth of many bacteria but does not selectively isolate *C. diphtheriae*.
**Option B:** *MacConkey agar* is selective for Gram-negative enteric bacteria and would not grow *C. diphtheriae*.
**Option C:** *Löeffler’s serum slope* is used for **preliminary enrichment** of *C. diphtheriae* but not definitive diagnosis.

**Clinical Pearl / High-Yield Fact**
Never confuse *Corynebacterium diphtheriae* with *Corynebacterium pseudodiphtheriticum

✓ Correct Answer: B. Potassium tellurite agar