**Core Concept**
Differential media are used to differentiate between organisms based on the way they grow or react to the medium, whereas selective media inhibit the growth of certain organisms. A differential medium may not necessarily inhibit the growth of unwanted organisms.

**Why the Correct Answer is Right**
Blood agar is an example of differential media that allows for the differentiation of bacteria based on their hemolytic activity. The medium contains sheep or horse blood, which is lysed by certain bacteria, resulting in a clear area around the colonies. This allows for the differentiation of bacteria based on their hemolytic activity, such as alpha, beta, or gamma hemolysis. Blood agar does not inhibit the growth of unwanted organisms, making it a differential medium but not selective.

**Why Each Wrong Option is Incorrect**
**Option A:** MacConkey agar is a selective and differential medium that inhibits the growth of gram-positive bacteria and differentiates between lactose-fermenting and non-lactose-fermenting gram-negative bacteria.

**Option B:** Eosin methylene blue agar is a selective and differential medium that inhibits the growth of gram-positive bacteria and differentiates between lactose-fermenting and non-lactose-fermenting gram-negative bacteria.

**Option D:** Mannitol salt agar is a selective medium that inhibits the growth of most gram-negative bacteria and allows the growth of gram-positive bacteria, such as Staphylococcus aureus.

**Clinical Pearl / High-Yield Fact**
Remember that differential media are used to differentiate between organisms based on their growth or reaction to the medium, whereas selective media inhibit the growth of certain organisms. This is an important distinction to make when choosing the appropriate medium for culturing bacteria.

**Correct Answer:** C. Blood agar is a differential medium that allows for the differentiation of bacteria based on their hemolytic activity, but it does not inhibit the growth of unwanted organisms.