**Core Concept:** Orbital cellulitis is a severe form of bacterial infection that affects the tissues surrounding the eyeball. It can lead to vision loss, brain abscess, or even death if not treated promptly. The infection can spread from the surrounding structures or be haematogenously spread.

**Why the Correct Answer is Right:** Orbital cellulitis is a complication of **D**, which refers to the spread of infection from the facial bones, sinuses, or teeth. Infections from the teeth (e.g., dental caries or dental abscess) can spread via the bloodstream or direct extension to involve the orbit.

**Why Each Wrong Option is Incorrect:**

A. **Orbital cellulitis** can be a complication of **C**, which is related to bacterial infections of the skin and soft tissues (e.g., cellulitis). However, orbital cellulitis is not a direct complication of skin infections.

B. **Orbital cellulitis** is not directly related to infections from the structures of the **A** (ears, nose, and throat) region. While these areas can contribute to the spread of infection, orbital cellulitis is more commonly linked to infections from the **D** region.

E. **Orbital cellulitis** is not a complication of infections from the **B** (upper respiratory tract) region. While respiratory tract infections can contribute to the spread of infection to the orbit, the correct answer (D) focuses on a more direct route of infection.

**Clinical Pearl:** Early recognition and prompt treatment of dental infections and sinusitis are crucial to prevent orbital cellulitis and its severe complications. In patients presenting with ocular pain, redness, swelling, and decreased vision, orbital cellulitis should be considered, and a thorough examination and imaging studies should be performed to identify the source of infection.