**Core Concept**
Ethmoidal polyps are a type of nasal polyp that originates from the ethmoid sinuses, a group of small air cells located within the ethmoid bone. These polyps are often associated with chronic inflammation and are typically seen in the context of chronic rhinosinusitis.

**Why the Correct Answer is Right**
Ethmoidal polyps are known to be recurrent, meaning they tend to regrow or reoccur after initial treatment. This is because the underlying inflammation and mucosal changes that cause the polyps to form are not fully addressed by treatment, leading to a cycle of growth and regression. The recurrent nature of these polyps is a key characteristic that distinguishes them from other types of nasal polyps. The exact mechanisms behind this recurrence are not fully understood, but it is thought to involve a combination of genetic predisposition, environmental factors, and the presence of underlying inflammatory conditions such as asthma or allergies.

**Why Each Wrong Option is Incorrect**
**Option A:** Infection is not the primary cause of ethmoidal polyps. While infection can trigger an inflammatory response that contributes to polyp formation, the underlying cause is typically a chronic inflammatory process rather than a direct infectious agent.
**Option B:** Ethmoidal polyps are often multiple, rather than single. They can be part of a larger polypoid mass that involves multiple sinuses and nasal passages.
**Option D:** While ethmoidal polyps can occur in children, they are more commonly seen in adults. In children, nasal polyps are often associated with cystic fibrosis or other genetic disorders.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the presence of recurrent nasal polyps can be a marker for underlying systemic diseases such as asthma or aspirin-exacerbated respiratory disease (AERD). Identifying these underlying conditions is critical for developing effective treatment plans and preventing long-term complications.

**✓ Correct Answer: C. Recurrent**