**Core Concept**
Acute epiglottitis is a medical emergency characterized by inflammation of the epiglottis, leading to airway obstruction. In children, the most common cause of acute epiglottitis is a bacterial infection.

**Why the Correct Answer is Right**
The correct answer is **Haemophilus influenzae type b (Hib)**. This bacterium is a leading cause of acute epiglottitis in children, particularly those under 5 years of age. Hib infections often present with sudden onset of high fever, dysphagia, and stridor. The infection can lead to significant airway compromise, requiring prompt medical attention. The epiglottis becomes swollen and red, potentially blocking the airway. Hib infections are usually caused by a specific capsular polysaccharide antigen, which can be targeted by conjugate vaccines.

**Why Each Wrong Option is Incorrect**
* **Option A:** Streptococcus pneumoniae is a common cause of pneumonia and meningitis but is not the most common cause of acute epiglottitis in children.
* **Option B:** Staphylococcus aureus can cause a range of infections, including skin and soft tissue infections, but is not typically associated with acute epiglottitis.
* **Option C:** Escherichia coli can cause urinary tract infections and neonatal sepsis but is not a common cause of acute epiglottitis in children.

**Clinical Pearl / High-Yield Fact**
The "thumb sign" is a classic radiographic finding in acute epiglottitis, where the swollen epiglottis appears as a thumb-like structure on lateral neck X-ray.

**Correct Answer: C. Haemophilus influenzae type b (Hib)**