## **Core Concept**
The question tests the understanding of rheumatological conditions, specifically those that present with back pain, joint pain, and radiographic evidence of sacroilitis. Sacroilitis refers to inflammation of the sacroiliac joints, which can be a hallmark of certain rheumatic diseases.

## **Why the Correct Answer is Right**
The correct answer, **Ankylosing Spondylitis (AS)**, is a type of seronegative spondyloarthropathy that characteristically presents with chronic back pain, stiffness, and limited mobility. A hallmark radiographic feature of AS is bilateral sacroilitis, which can progress to syndesmophyte formation and spinal fusion. The patient's symptoms of back pain and multiple joint pains, along with the X-ray finding of bilateral sacroilitis, are highly suggestive of AS.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Rheumatoid Arthritis (RA)** can cause joint pain and inflammation, it typically does not present with sacroilitis. RA more commonly affects the small joints of the hands and feet and is usually associated with positive rheumatoid factor and anti-CCP antibodies.
- **Option B:** **Psoriatic Arthritis (PsA)** can cause joint pain and can be associated with sacroilitis, but it typically occurs in the context of psoriasis. The question does not provide information about skin symptoms.
- **Option D:** **Gout** primarily presents with acute, episodic monoarthritis, commonly affecting the base of the big toe. While gout can cause joint pain, it is not typically associated with sacroilitis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Ankylosing Spondylitis** often presents in young adults but can be diagnosed at any age. The presence of bilateral sacroilitis on X-ray is a diagnostic criterion for AS, and the disease is strongly associated with the HLA-B27 genotype.

## **Correct Answer: C. Ankylosing Spondylitis.**