**Question:** A 17-year-old boy presents with chronic low back pain and stiffness for the past 8 months. He denies any GIT or genital infections. His temp is 98.6 F. Lab investigation shows normal ESR and negative RA factor. X-ray films of the vertebral column show flattening of the lumbar curve and subchondral bone erosion involving the sacroiliac joints. Most likely diagnosis?

A. Ankylosing spondylitis
B. Reactive arthritis
C. Osteoarthritis
D. Psoriatic arthritis

**Core Concept:**
The provided question is centered around the differential diagnosis of a patient presenting with chronic low back pain, stiffness, and specific radiological findings. The key factors to consider include the age of onset, duration of symptoms, and associated findings. The correct diagnosis depends on the correct interpretation of these clinical and radiological findings.

**Why the Correct Answer is Right:**
Given the patient's age (17 years), the pain duration (8 months), and the described radiological findings (flattening of lumbar curve and subchondral bone erosion involving sacroiliac joints), the most likely diagnosis is **Ankylosing spondylitis (AS)**.

**Why Each Wrong Option is Incorrect:**

A. **Ankylosing spondylitis (AS)** is a type of inflammatory arthritis that primarily affects the spine and sacroiliac joints. The age of onset is typically during adolescence or early adulthood, which is not the case in our patient (17 years old).

B. **Reactive arthritis (ReA)** is a type of inflammatory arthritis resulting from an infection (usually gastrointestinal or genitourinary) and is not the correct diagnosis for our patient without any history of infections.

C. **Osteoarthritis (OA)** is a degenerative joint disease caused by wear and tear, which is also not the correct diagnosis for our patient with inflammatory signs and symptoms.

D. **Psoriatic arthritis (PA)** is a type of inflammatory arthritis associated with skin lesions and psoriasis, which is not present in our patient.

**Clinical Pearl:**

Ankylosing spondylitis is a type of inflammatory arthritis affecting the spine and sacroiliac joints, presenting in young adults. In contrast, our patient is 17 years old, which makes AS less likely.

**Why the Correct Answer is Right:**

Ankylosing spondylitis is an inflammatory arthritis affecting the spine and sacroiliac joints, leading to back pain, stiffness, and spinal deformity. The radiographic features of AS include the presence of bowing of the spine (Beck's sign), sacroiliac joint inflammation (Perplexitis), and spinal fusion (Bambrick's sign).

**Why Each Wrong Option is Incorrect:**

Reactive arthritis (option B) is a distinct entity caused by an infection, which is not the case for our patient with no history of infection.

Osteoarthritis (option C) is a degenerative joint disease resulting from wear