**Question:** A 5-year-old child with recurrent sinusitis, cause is:

A. Allergic rhinitis
B. Congenital anomalies of the nasal cavity
C. Immunodeficiency disorders
D. Post-infectious sinusitis

**Core Concept:**
Recurrent sinusitis in a 5-year-old child is a clinical condition characterized by inflammation of the sinuses. The etiology can be broadly classified into intrinsic factors (inherent to the child) and extrinsic factors (environmental factors). In this question, we are considering intrinsic causes.

**Why the Correct Answer is Right:**
Recurrent sinusitis in a 5-year-old child should be primarily considered in the context of immunodeficiency disorders. These conditions can lead to a compromised immune system, making the child more susceptible to infections and inflammation in the sinuses. Among the given options, immunodeficiency disorders are the intrinsic factors that can cause recurrent sinusitis in a child.

**Why Each Wrong Option is Incorrect:**
1. Allergic rhinitis (Option A) is an extrinsic factor, as it is an allergy-related condition. While allergies can exacerbate sinusitis, they are not intrinsic causes for the recurrent nature in a 5-year-old child.
2. Congenital anomalies of the nasal cavity (Option B) are rare and not a common intrinsic cause for recurrent sinusitis in young children. Instead, these anomalies are usually diagnosed during childhood or adolescence.
3. Post-infectious sinusitis (Option D) occurs following an acute sinus infection, but it is not an intrinsic cause in a 5-year-old child. The condition typically occurs in adults after a sinus infection, not in children.

**Clinical Pearl:**
Recurrent sinusitis in a 5-year-old child should raise suspicion for underlying immunodeficiency disorders. These conditions should be investigated thoroughly, including laboratory tests and imaging studies, to identify the cause and initiate appropriate management. Early diagnosis and treatment can prevent complications and improve the child's quality of life.