**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, characterized by the production of a toxin that leads to tissue necrosis and membrane formation. The earliest growth of diphtheria can be detected on a specific type of agar medium due to its unique colonial morphology.

**Why the Correct Answer is Right**
Corynebacterium diphtheriae exhibits a characteristic "metachromatic" growth on Tellurite agar, which contains tellurite as a selective agent. The bacteria reduce the tellurite to metallic tellurium, resulting in a dark brown or black pigment on the surface of the agar. This distinctive coloration is a diagnostic feature of C. diphtheriae. The metachromatic property is due to the presence of polyphosphates in the bacterial cell wall, which react with the tellurite to form the visible pigment.

**Why Each Wrong Option is Incorrect**
* **Option A:** Loeffler's serum slope is used for the isolation of C. diphtheriae, but it is not the medium where the earliest growth is detected.
* **Option B:** Blood agar is a general-purpose medium for growing a variety of bacteria, but it is not selective for C. diphtheriae.
* **Option C:** MacConkey agar is used for the isolation of Gram-negative bacteria, particularly those that ferment lactose, and is not relevant for detecting C. diphtheriae.

**Clinical Pearl / High-Yield Fact**
It is essential to note that C. diphtheriae can also be isolated on Loeffler's serum slope, where it exhibits a characteristic "ragged" or "cotton ball" growth. However, the earliest growth is typically detected on Tellurite agar.

**Correct Answer: C. Tellurite agar.**