## **Core Concept**
Corynebacterium diphtheriae is a Gram-positive bacterium that causes diphtheria. The isolation and cultivation of this bacterium require specific media that support its growth and, ideally, help in differentiating it from other bacteria.

## **Why the Correct Answer is Right**
The correct answer, **Tellurite agar**, is a selective medium used for the isolation of Corynebacterium diphtheriae. This medium contains potassium tellurite, which inhibits the growth of many other bacteria while allowing C. diphtheriae to grow. C. diphtheriae can reduce tellurite, leading to the formation of black colonies. This property makes tellurite agar particularly useful for the isolation and preliminary identification of C. diphtheriae.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although not specified, common media like blood agar or nutrient agar do not selectively inhibit other bacteria's growth while allowing C. diphtheriae to grow with distinctive characteristics.
- **Option B:** This option might refer to a medium like MacConkey agar, which is selective for Gram-negative bacteria and not suitable for C. diphtheriae.
- **Option C:** Without specifics, it's hard to address directly, but generally, media like chocolate agar are used for fastidious organisms like Haemophilus and Neisseria, not C. diphtheriae.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Corynebacterium diphtheriae** requires **specific media** like tellurite agar for isolation. This bacterium is significant because it causes diphtheria, a potentially life-threatening infection that primarily affects the respiratory system. Early identification through culture is crucial for initiating appropriate antibiotic therapy.

## **Correct Answer:** D. Tellurite agar.