**Question:** A 23-year-old man notices a gradual but progressive increase in breathing difficulty. He has a long history of back pain with prolonged morning stiffness. He has also had an episode of iritis in the past. On examination, there is reduced range of motion in the lumbar spine with forward flexion and pain on palpation of the sacroiliac joint and surrounding soft tissue. X-rays of the pelvis show erosions and sclerosis of the sacroiliac joint. Which of the following is the most likely pulmonary complication of this condition?

A. Chronic bronchitis
B. Pulmonary fibrosis
C. Chronic obstructive pulmonary disease (COPD)
D. Pneumonia

**Correct Answer:** C. Chronic obstructive pulmonary disease (COPD)

**Core Concept:** Chronic obstructive pulmonary disease (COPD) is a group of lung diseases characterized by irreversible airflow obstruction, progressive respiratory symptoms, and exacerbations. In this case, the patient presents with a history of longstanding back pain, morning stiffness, reduced range of motion in the lumbar spine, and an episode of iritis in the past. Additionally, the X-rays of the pelvis show erosions and sclerosis of the sacroiliac joint. These clinical features are suggestive of ankylosing spondylitis (AS) which is a type of inflammatory arthritis primarily affecting the spine and sacroiliac joints.

**Why the Correct Answer is Right:** The most likely pulmonary complication in this scenario is Chronic Obstructive Pulmonary Disease (COPD). COPD is a common complication in patients with ankylosing spondylitis (AS) due to the following reasons:

1. **Ankylosing Spondylitis (AS) is a chronic inflammatory disease affecting the spine and sacroiliac joints.** The inflammation in AS can lead to lung involvement as a result of the migration of inflammatory cells into the lung parenchyma, and the formation of granulomas and fibrosis in the lung parenchyma and pleura. This can cause respiratory symptoms such as dyspnea, cough, and chest pain.

2. **Inflammatory arthritis in AS can lead to lung complications.** The inflammation in AS can cause parenchymal fibrosis, pleural fibrosis, and interstitial lung disease, which can lead to respiratory symptoms and a decline in lung function. These complications are more common in patients with established AS.

**Why Other Answers are Incorrect:**

1. **Chronic bronchitis (Option A):** This refers to inflammation of the airways and bronchioles, causing cough, sputum production, and wheezing. However, in this case, the patient presents with respiratory symptoms like dyspnea and chest pain.

2. **Pulmonary fibrosis (Option B):** This condition refers to the scarring of lung tissue, causing breathlessness, dry cough, and pleural thickening. Although pulmonary fibrosis can occur in AS patients, the provided symptoms are more consistent with AS-related respiratory complications.

3. **Pneumonia (Option D):** Pneumonia is an infection affecting the lungs, causing fever, cough, and chest pain. In this case, the