**Core Concept:** Ankylosing spondylitis (AS) is a chronic inflammatory disorder primarily affecting the axial skeleton, particularly the spine and sacroiliac joints. It is a type of seronegative spondyloarthropathy, meaning it does not test positive for rheumatoid factor.

**Why the Correct Answer is Right:** Ankylosing spondylitis primarily affects the spine, leading to progressive fusion of the vertebrae, particularly at the lumbosacral junction, which results in stiffness, pain, and limited mobility. This manifestation is the most common extra-articular manifestation, as it primarily involves the joints and bones.

**Why Each Wrong Option is Incorrect:**
A. Arthritis of peripheral joints: AS primarily affects axial joints, not peripheral joints like the wrists, knees, or ankles.
B. Inflammatory bowel disease: While AS can be associated with inflammatory bowel disease, the correct answer focuses on the spinal manifestations of AS.
C. Cardiac involvement: AS rarely causes cardiac manifestations, making it an unlikely correct answer.
D. Ophthalmologic involvement: While AS can cause uveitis and other eye issues, the correct answer focuses on spinal manifestations.

**Why the Correct Answer is Right:**

The correct answer, **C.** Behçet's disease, is a chronic inflammatory disorder affecting the skin, eyes, oral mucosa, and genitalia, as well as the gastrointestinal, central nervous, and cardiovascular systems. It is characterized by recurrent oral ulcers, genital ulcers, and ocular lesions. The disease is named after Hulusi Behçet, a Turkish dermatologist who first described the syndrome in 1937.

**Why Each Wrong Option is Incorrect:**
A. Erythema nodosum: This is a skin condition, not related to spinal manifestations.
B. Reactive arthritis: This condition primarily affects the joints, not the spinal manifestations.
C. Behçet's disease: Spinal involvement is the primary manifestation of Behçet's disease, making it the correct answer.
D. Ankylosing spondylitis: This is a different condition that primarily affects the spine, not the peripheral joints.
E. Polyarteritis nodosa: This is a vasculitis affecting small and medium-sized arteries, not spinal manifestations.

**Clinical Pearl:** Behçet's disease is essential to differentiate from ankylosing spondylitis, Reactive arthritis, and other related conditions, as this can guide appropriate clinical management and prognosis.