**Core Concept**
Hyperammonemia refers to an elevated level of ammonia in the blood, which can be caused by various factors including metabolic disorders, liver disease, or certain medications. Ammonia is a toxic compound that can cause neurological symptoms and even coma if not treated promptly.

**Why the Correct Answer is Right**
One of the medications that can cause hyperammonemia is valproic acid, a commonly used anticonvulsant and mood stabilizer. Valproic acid can increase the production of ammonia by inhibiting the enzyme mitochondrial carnitine palmitoyltransferase 1A (CPT1A), which is involved in the breakdown of fatty acids. This leads to an accumulation of ammonia in the blood. Additionally, valproic acid can also impair the function of the urea cycle, further contributing to hyperammonemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** While certain antiviral medications like valganciclovir can cause hyperammonemia, it is not the most commonly associated medication with this side effect.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
When prescribing valproic acid, it is essential to monitor liver function tests and ammonia levels regularly, especially in patients with pre-existing liver disease or those taking other medications that can exacerbate hyperammonemia.

**Correct Answer:** B.