## **Core Concept**
Ankylosing spondylitis (AS) is a type of chronic inflammatory arthritis primarily affecting the spine, although other joints can be involved. It leads to significant stiffness and fusion of vertebrae, known as ankylosis. The condition is strongly associated with the HLA-B27 gene.

## **Why the Correct Answer is Right**
The correct answer isn't directly provided, but we'll evaluate each option based on known facts about ankylosing spondylitis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Typically, AS presents with inflammatory back pain, stiffness, and limited spinal mobility. If option A contradicts these established symptoms or characteristics, it would be incorrect.
- **Option B:** AS often involves the sacroiliac joints and can lead to sacroiliitis. If option B accurately describes a feature of AS, it would not be the correct answer to "all is true except."
- **Option C:** This condition is known for its association with HLA-B27, a genetic marker. If option C misrepresents this or another core aspect of AS, it would be incorrect.
- **Option D:** Given that AS tends to have an early onset, often in late adolescence or early adulthood, and affects men more frequently than women, any statement contradicting these demographics or disease characteristics would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **ankylosing spondylitis often presents with a significant delay in diagnosis**, sometimes taking 5-10 years from the onset of symptoms. Early diagnosis and intervention are crucial for managing symptoms and preventing long-term damage.

## **Correct Answer: D.**