**Core Concept**
Atrophic rhinitis is a chronic inflammatory condition characterized by atrophy of the nasal mucosa, leading to a reduction in the size of the nasal cavity and impairment of the mucociliary clearance mechanism. The condition can result in a significant decrease in the production of mucous membranes, creating an environment conducive to obstruction.

**Why the Correct Answer is Right**
The most common cause of obstruction in atrophic rhinitis is **crust formation**. This occurs due to the decreased production of mucous membranes and the increased presence of blood, serum, and cellular debris in the nasal cavity. The crusts formed can be quite hard and adhere to the nasal mucosa, causing significant obstruction.

**Why Each Wrong Option is Incorrect**
* **Option A:** Nasal polyps are a common cause of obstruction in various forms of rhinitis, but not typically in atrophic rhinitis.
* **Option B:** Hypertrophy of nasal turbinate bones can cause nasal obstruction, but it is not directly related to atrophic rhinitis.
* **Option D:** Allergic rhinitis can cause nasal obstruction due to the release of histamine and other chemical mediators, but it is not the most common cause in atrophic rhinitis.

**Clinical Pearl / High-Yield Fact**
Crusts in atrophic rhinitis can be quite hard and may require gentle removal to avoid causing further trauma to the nasal mucosa. It is essential to treat the underlying condition to prevent the recurrence of crusts.

**Correct Answer: C. Crust formation. Crust formation.**