## **Core Concept**
The patient's symptoms of backache that occurs early in the morning, stiffness, and limited chest expansion suggest an inflammatory cause, particularly a type of spondyloarthritis. This condition affects the spine and can lead to significant morning stiffness and reduced mobility.

## **Why the Correct Answer is Right**
The correct answer, **Ankylosing Spondylitis (AS)**, is a form of spondyloarthritis characterized by chronic inflammation of the spine and the sacroiliac joints. The symptoms described, such as morning stiffness, backache that persists for more than an hour, and reduced chest expansion, are classic for AS. This condition often presents in young adults and can lead to significant disability if not treated promptly. The inflammation in AS can cause the vertebrae to fuse, leading to a loss of spinal mobility.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Mechanical Low Back Pain** is common, it typically does not present with prolonged morning stiffness or significantly reduced chest expansion. It is more related to physical strain or posture.
- **Option B:** **Degenerative Disc Disease** involves wear and tear on the spinal discs and can cause back pain, but it usually doesn't cause significant morning stiffness lasting more than an hour or reduced chest expansion to the extent seen in inflammatory conditions.
- **Option D:** **Psoriatic Arthritis** can cause back pain and stiffness but is typically associated with skin psoriasis and often involves the distal joints of the hands and feet, which are not mentioned in the scenario.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Ankylosing Spondylitis** often presents with **Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) score** and **chest expansion** as critical diagnostic and monitoring tools. Early diagnosis and treatment can significantly improve outcomes.

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