**Question:** All the following can cause mouth ulcers except ?

A. Trauma
B. Vitamin B12 deficiency
C. Infections (herpes simplex virus, human herpesvirus-8)
D. Autoimmune disorders (systemic lupus erythematosus)

**Core Concept:** Mouth ulcers are painful lesions that occur in the oral cavity, primarily affecting the mucous membranes and lips. They can be caused by various factors, including trauma, infections, and autoimmune disorders.

**Why the Correct Answer is Right:** Vitamin B12 deficiency is not a direct cause of mouth ulcers but can lead to other symptoms such as anemia and neurological complications. Autoimmune disorders like systemic lupus erythematosus also cause systemic involvement, affecting various organs but not specifically causing mouth ulcers.

**Why Each Wrong Option is Incorrect:**

A. Trauma: Trauma, including biting, brushing forcefully, or sharp foods, can directly damage the oral mucosa and lead to mouth ulcers.
B. Vitamin B12 deficiency: Although less common, vitamin B12 deficiency can lead to anemia and neurological symptoms, but not specifically cause oral ulcers.
C. Infections: Infections, particularly herpes simplex virus and human herpesvirus-8, can directly infect the oral mucosa and induce mouth ulcers.
D. Autoimmune disorders: Autoimmune disorders like systemic lupus erythematosus can cause systemic involvement, affecting various organs, but not specifically leading to mouth ulcers.

**Clinical Pearl:** Mouth ulcers are often caused by direct trauma, infections, or specific vitamin deficiencies. Vitamin B12 deficiency typically presents with more systemic symptoms, and autoimmune disorders usually involve multiple organs. Infectious causes, such as viral infections, are the primary culprits for mouth ulcers.