Okay, let's tackle this question. The patient is a 27-year-old male with low back pain that's morning-related and lasts over 30 minutes. Also, chest expansion is restricted. Hmm, so I need to think about the possible diagnoses here.

First, the key symptoms are inflammatory back pain characteristics. Morning stiffness lasting more than 30 minutes is classic for inflammatory arthritis. The restricted chest expansion is a red flag. Common conditions that come to mind are axial spondyloarthritis, like ankylosing spondylitis. That's a HLA-B27 associated condition. It affects the spine and can lead to chest wall restriction due to costovertebral joint involvement.

Other possibilities: maybe mechanical back pain, but that usually doesn't have the morning stiffness for that long. Osteoarthritis is more common in older people. Maybe a malignancy? But 27 is young. Tuberculosis? Could be, but usually presents with other systemic symptoms.

Wait, the question mentions the chest expansion is restricted. Ankylosing spondylitis can cause fusion of the spine, leading to limited chest expansion. That's a key point. So the most probable diagnosis here is ankylosing spondylitis. The options probably include that. Let me check the options again. The correct answer would be ankylosing spondylitis. The other options would be other causes, which don't fit the clinical picture as well.

So the core concept is recognizing the cardinal features of ankylosing spondylitis. The correct answer is based on the typical presentation. The wrong options don't fit the chronic, inflammatory, and systemic features. The clinical pearl is that chest expansion restriction is a key sign in AS.

**Core Concept**
This question tests recognition of **ankylosing spondylitis (AS)**, a chronic **inflammatory spondyloarthropathy** targeting the axial skeleton. Key features include **inflammatory back pain**, **morning stiffness >30 minutes**, and **extra-articular manifestations** like **restricted chest expansion** due to costovertebral joint involvement.

**Why the Correct Answer is Right**
Ankylosing spondylitis is the most likely diagnosis due to the **classic triad** of:
1. **Chronic inflammatory back pain** (onset 30 minutes).
2. **Axial involvement** (sacroiliitis, vertebral syndesmophytes).
3. **Extra-articular features** (restricted chest expansion from costovertebral fusion).
HLA-B27 positivity is common but not diagnostic. The condition leads to **kyphosis** and **reduced pulmonary function** due to chest wall rigidity.

**Why Each Wrong Option is Incorrect**
**Option A:** *Degenerative disc disease* causes mechanical back pain, not inflammatory stiffness or chest restriction.
**Option B:** *Osteoarthritis* typically affects older adults and lacks systemic inflammation or chest involvement.
**Option C:** *Tuberculosis of the spine* (Pott’s disease) causes destructive lesions, weight loss, and fever—unrelated to chest expansion.

**Clinical Pearl / High-Yield Fact**
**"Stiff as a board"** in AS leads to **impaired chest expansion**

✓ Correct Answer: D. Ankylosing spondylitis