Okay, let's tackle this question. The patient is a 22-year-old male with low back pain and stiffness that's getting worse over months, especially at night. He has hip pain too. On exam, there's paravertebral muscle tenderness and limited lumbar flexion. The x-ray is mentioned but not provided here, but the key here is to think about the possible causes of chronic back pain in a young adult.

First, I need to consider the common conditions in this age group. The classic red flags for back pain include mechanical issues, infections, tumors, and inflammatory conditions. Since this is a young man, inflammatory arthritis comes to mind. Ankylosing spondylitis (AS) is a type of spondyloarthritis that typically affects the spine and sacroiliac joints. It's more common in males and often presents with insidious onset of back pain and stiffness, worse at night. The presence of limited lumbar flexion and paraspinal tenderness are signs of chronic inflammation. The x-ray would show changes like sacroiliitis, vertebral squaring, and eventually bamboo spine in advanced cases.

Other possibilities could be mechanical low back pain, but that usually doesn't have the stiffness and night pain. Maybe osteoarthritis, but that's more common in older people. Paget's disease? Unlikely in a 22-year-old. Maybe a tumor or infection, but those would present with more acute or severe symptoms and systemic signs.

The key here is the age, gender, symptoms of stiffness and night pain, and the chronic progression. AS is the most likely diagnosis. The x-ray would show specific findings like sacroiliitis, which is a hallmark. The correct answer would be ankylosing spondylitis.

**Core Concept**
This case tests recognition of inflammatory back pain patterns and imaging features of ankylosing spondylitis (AS). AS is a seronegative spondyloarthropathy characterized by sacroiliitis and axial joint inflammation, commonly presenting in young males with insidious onset of stiffness and night pain.

**Why the Correct Answer is Right**
Ankylosing spondylitis is the most likely diagnosis because the clinical presentation—chronic low back pain, night stiffness, hip pain, and limited lumbar flexion—aligns with axial spondyloarthritis. The x-ray would classically show sacroiliitis (early erosion, sclerosis) or vertebral squaring. AS is HLA-B27 associated and follows a relapsing-remitting course with potential fusion of the spine ("bamboo spine") in advanced stages. The paravertebral tenderness reflects inflammatory enthesopathy.

**Why Each Wrong Option is Incorrect**
**Option A:** Degenerative disc disease typically presents in older adults with acute mechanical pain, not chronic stiffness or night symptoms.
**Option B:** Lumbar strain causes acute pain with muscle spasm but resolves within weeks, not months.
**Option C:** Osteoarthritis of the spine occurs in older individuals and involves facet joint degeneration, not systemic inflammation.

**Clinical Pearl / High-Yield Fact**
Never forget the "4 pillars" of AS: **A**xial pain, **S**acroiliitis (on imaging), **S**pondyloarthritis (inflammatory), and **

✓ Correct Answer: C. ankylosing spondylitis (AS)