**Question:** Which of the following agent of meningitis can grow on chocolate agar but not on blood agar?

A. Streptococcus pneumoniae
B. Haemophilus influenzae
C. Neisseria meningitidis
D. Lactobacillus species

**Correct Answer:** C. Neisseria meningitidis

**Core Concept:**
Neisseria meningitidis, also known as meningococcus, is a gram-negative diplococcus that causes meningitis, a severe and potentially life-threatening infection of the protective membranes covering the brain and spinal cord. Meningococci are known for their ability to produce certain enzymes that allow them to survive and grow on specific agar media.

**Why the Correct Answer is Right:**
Neisseria meningitidis is a selective pathogen that can be distinguished from other bacteria based on its unique physiological characteristics. Chocolate agar is a selective medium designed to support the growth of hemolytic bacteria (capable of breaking down red blood cells), which is essential for N. meningitidis since it is a facultative anaerobe and requires oxygen for growth. Blood agar, on the other hand, is a general-purpose agar medium that supports the growth of a wide variety of bacteria, including many that do not require hemolysis for growth, such as Streptococcus pneumoniae (A) and Haemophilus influenzae (B).

**Why Each Wrong Option is Incorrect:**

A. Streptococcus pneumoniae (A) cannot grow on chocolate agar because it is a facultative anaerobe, not a strict anaerobe like N. meningitidis. Streptococcus pneumoniae prefers to grow on blood agar due to its aerobic nature.

B. Haemophilus influenzae (B) is a facultative anaerobe, similar to N. meningitidis. However, H. influenzae lacks the necessary enzymes for hemolysis, which is essential for growth on chocolate agar. Thus, it prefers to grow on blood agar.

C. Lactobacillus species (D) are lactic acid bacteria that are generally considered gram-positive and anaerobic. These microorganisms cannot cause meningitis and are not relevant to the question.

**Clinical Pearl:**
Chocolate agar is a valuable tool in clinical microbiology for the differentiation of meningococcal strains, as it specifically supports the growth of N. meningitidis due to its requirement for oxygen and the presence of catalase enzyme. This allows for a more accurate identification of the causative agent in a patient with meningitis, facilitating appropriate treatment and reducing the risk of false-negative results due to competing growth of other bacteria.