Final answer:
The most likely suspect for the 22-year-old patient's symptoms is ankylosing spondylitis, a type of arthritis that primarily affects the spine and sacroiliac joints.
Step-by-step explanation:
The most likely suspect for the 22-year-old patient's symptoms of gradual onset of thoracic and sacroiliac pain, limited lumbar range of motion (ROM), and worsened stiffness in the morning is ankylosing spondylitis. Ankylosing spondylitis is a type of arthritis that primarily affects the spine and sacroiliac joints, leading to inflammation, pain, and stiffness. The symptoms often worsen after periods of rest, such as in the morning, and improve with movement and exercise.
In this case, the patient's long periods of sitting at a new job may contribute to the pain and stiffness, as extended periods of inactivity can aggravate symptoms of ankylosing spondylitis. Additionally, the age of onset (22 years old) aligns with the typical age range for developing ankylosing spondylitis.