**Question:** Which one of the following micro-organisms uses antigenic variation as a major means of variation as an evading host defences?

A. Plasmodium falciparum
B. Streptococcus pneumoniae
C. Trypanosoma brucei
D. Neisseria meningitidis

**Correct Answer:** C. Trypanosoma brucei

**Core Concept:** Antigenic variation is a strategy employed by certain microorganisms to evade the host immune system. This mechanism allows the organism to alter its surface antigens, making it difficult for the host immune system to recognize and target the pathogen consistently.

**Why the Correct Answer is Right:** Trypanosoma brucei, a protozoan parasite responsible for African sleeping sickness, employs antigenic variation as a crucial evasion mechanism. When the host immune system mounts an attack against specific antigens, T. brucei can rapidly switch its surface antigens, preventing a targeted immune response. This characteristic allows the parasite to survive and persist in the host for an extended period.

**Why Each Wrong Option is Incorrect:**

A. Plasmodium falciparum, responsible for malaria, does not rely on antigenic variation as a primary immune evasion strategy. Instead, it utilizes other mechanisms such as sequestration in microvascular beds and modulation of host immune response.

B. Streptococcus pneumoniae, causing pneumonia, primarily relies on capsular polysaccharide variation as a means to evade the host immune system. Antigenic variation is less commonly involved in its survival strategies.

D. Neisseria meningitidis, responsible for meningitis, also primarily employs capsular polysaccharide variation to evade the host immune system. Antigenic variation does not play a significant role in its survival strategies.

**Clinical Pearl:** Understanding the diverse immune evasion strategies employed by pathogens is crucial for effectively treating and managing infections caused by these organisms. In addition, recognizing the specific mechanisms employed by each microorganism can aid in developing targeted vaccines and treatments.