**Core Concept**
Corynebacterium diphtheriae and Corynebacterium ulcerans are two closely related species that can cause diphtheria, a serious bacterial infection. The biochemical differentiation between these two species is crucial for accurate diagnosis and treatment.

**Why the Correct Answer is Right**
Corynebacterium diphtheriae and Corynebacterium ulcerans can be differentiated based on their ability to ferment different sugars. C. diphtheriae can ferment glycerol, while C. ulcerans cannot. This is due to the presence of the glycerol dehydrogenase enzyme in C. diphtheriae, which is responsible for the fermentation of glycerol. On the other hand, C. ulcerans lacks this enzyme and therefore cannot ferment glycerol.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct biochemical property that differentiates C. diphtheriae from C. ulcerans.
**Option B:** This option is incorrect because it is not a valid biochemical property that can be used to differentiate between the two species.
**Option C:** This option is incorrect because it is not a correct biochemical property that can be used to differentiate between the two species.

**Clinical Pearl / High-Yield Fact**
It is essential to note that C. ulcerans is a more virulent strain than C. diphtheriae and can cause severe diphtheria, especially in individuals who are not vaccinated. Therefore, accurate identification of the causative organism is crucial for appropriate treatment and management.

**Correct Answer: A. Glycerol fermentation**