**Core Concept**
Sodium valproate is an anticonvulsant and mood stabilizer used in the treatment of bipolar mood disorder. It works by modulating the activity of various neurotransmitters and ions, including sodium, calcium, and potassium, which are crucial in maintaining neuronal excitability.

**Why the Correct Answer is Right**
Sodium valproate can cause weight gain, which is a common side effect due to its effect on the hypothalamic-pituitary-adrenal axis and the increase in leptin levels. This leads to an increase in appetite and food intake, resulting in weight gain. Additionally, sodium valproate can also cause polycystic ovary syndrome (PCOS) in some patients, which is characterized by hyperandrogenism, ovulatory dysfunction, and polycystic ovaries.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as sodium valproate does not commonly cause liver failure as a side effect. While it can cause liver enzyme elevations, liver failure is a rare but serious side effect.
**Option B:** This option is incorrect as sodium valproate does not commonly cause hypothyroidism as a side effect. Although it can affect thyroid function, hypothyroidism is not a typical side effect.
**Option C:** This option is incorrect as sodium valproate does not commonly cause agranulocytosis as a side effect. Although it can cause bone marrow suppression, agranulocytosis is a rare and serious side effect.

**Clinical Pearl / High-Yield Fact**
Sodium valproate is known to cause teratogenic effects, particularly neural tube defects, if taken during pregnancy. Therefore, it is essential to monitor for signs of teratogenicity and ensure that women of childbearing age are on adequate contraception while taking this medication.

**Correct Answer: A. Weight gain.**