**Question:** All are true regarding optic neuritis, EXCEPT:

A. Optic neuritis is an inflammation of the optic nerve, causing visual disturbances.
B. It is a common symptom of multiple sclerosis.
C. It is caused by a primary viral infection.
D. The treatment involves immunosuppressive drugs.

**Correct Answer: C.**

**Core Concept:** Optic neuritis is a medical condition characterized by inflammation of the optic nerve, leading to visual disturbances. This condition can have various causes and is often associated with immune-mediated disorders such as multiple sclerosis (MS), but can also result from other factors like infections, trauma, or tumors.

**Why the Correct Answer is Right:** Optic neuritis is indeed a common symptom of multiple sclerosis (MS), a chronic inflammatory disease that affects the central nervous system (CNS). In MS, the immune system attacks the myelin sheath surrounding nerve fibers, causing inflammation and demyelination. This leads to various neurological symptoms, including optic neuritis.

**Why Each Wrong Option is Incorrect:**

A. While optic neuritis is indeed an inflammation of the optic nerve, stating that it is caused by a primary viral infection is incorrect. Optic neuritis can be caused by various factors, but viral infections are not the primary cause for most cases.

B. Although optic neuritis is associated with multiple sclerosis, stating that the treatment involves immunosuppressive drugs is not entirely accurate. The primary treatment for optic neuritis involves corticosteroids to reduce inflammation, whereas immunosuppressive drugs are used in the management of MS itself.

C. This option is incorrect because optic neuritis is caused by a variety of factors, some of which are not directly related to viral infections. While viral infections can lead to optic neuritis, not all cases are caused by primary viral infections.

**Clinical Pearl:** Optic neuritis is a crucial clinical manifestation to consider in a patient presenting with visual disturbances. It is essential to differentiate between primary viral infections and secondary optic neuritis due to other causes like MS, as they require distinct management strategies. A thorough history, physical examination, and appropriate investigations are necessary to establish the correct diagnosis and initiate appropriate treatment.