**Question:** C. diphtheriae can be biochemically differentiated from C. pseudotuberculosis by

A. fermentation of glucose
B. production of urease
C. hydrolysis of casein
D. fermentation of lactose

**Core Concept:** Biochemical differentiation is a process used in microbiology to distinguish between closely related microorganisms based on their unique biochemical reactions. In this case, we are comparing Corynebacterium diphtheriae and Corynebacterium pseudotuberculosis, two bacterial species that may exhibit similar characteristics due to their classification within the Corynebacterium genus.

**Why the Correct Answer is Right:** The correct answer, **D. fermentation of lactose**, refers to the ability of C. diphtheriae to ferment lactose, which means it can break down lactose into its constituent sugars (glucose and galactose) through the action of the enzyme lactase. This unique biochemical reaction differentiates C. diphtheriae from C. pseudotuberculosis, which does not possess this enzyme and thus does not ferment lactose.

**Why Each Wrong Option is Incorrect:**

A. **Fermentation of glucose (C. pseudotuberculosis)** - This is incorrect because C. pseudotuberculosis can ferment glucose, making it a wrong option for differentiation between the two species.

B. **Production of urease (C. diphtheriae)** - This is incorrect because C. diphtheriae does produce urease, which is a urea-hydrolyzing enzyme. Urease is present in C. diphtheriae but not in C. pseudotuberculosis, making it a wrong option for differentiation.

C. **Hydrolysis of casein (C. pseudotuberculosis)** - This is incorrect because C. pseudotuberculosis has the ability to hydrolyze casein, a milk protein. Casein hydrolysis is a characteristic of C. pseudotuberculosis, making it a wrong option for differentiation between the two species.

**Clinical Pearl:** Understanding the biochemical differences between microorganisms is essential for accurate identification and appropriate treatment of infections caused by these organisms. In this case, knowing the ability of C. diphtheriae to ferment lactose and lack of urease production, along with the ability of C. pseudotuberculosis to hydrolyze casein, can help healthcare professionals differentiate between these closely related species and choose the correct treatment options.