**Core Concept**
Diphtheria is an acute bacterial infection caused by Corynebacterium diphtheriae, characterized by the formation of a thick, gray membrane in the throat. The disease primarily affects children, but its incidence has decreased significantly due to widespread vaccination.

**Why the Correct Answer is Right**
The correct answer is related to the age group most susceptible to diphtheria, which is typically children under the age of 5 years. This is because young children have not developed sufficient immunity to the bacteria and are more likely to contract the disease. Vaccination plays a crucial role in preventing diphtheria, and children who are not up-to-date with their vaccinations are at higher risk of developing the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This age group is not the most susceptible to diphtheria. While older adults can still contract the disease, their immune systems may mount a more effective response to the infection.
* **Option B:** This age group is not typically associated with diphtheria. While adolescents can still contract the disease, the risk is lower compared to young children.
* **Option C:** This age group is not the most susceptible to diphtheria. While older adults can still contract the disease, their immune systems may mount a more effective response to the infection.

**Clinical Pearl / High-Yield Fact**
Diphtheria is a vaccine-preventable disease, and vaccination is the most effective way to prevent it. Children should receive a series of vaccinations against diphtheria, tetanus, and pertussis (DTP) to build immunity against these diseases.

**Correct Answer:** C. 5-14 years