**Core Concept**
Multiple carboxylase deficiency is a rare genetic disorder affecting the metabolism of certain amino acids, resulting from a deficiency in 2-methylacetoacetyl-CoA thiolase (MAT) or branched-chain alpha-keto acid dehydrogenase complex (BCKDC). This condition is characterized by impaired activity of these enzymes, leading to the accumulation of toxic metabolites.

**Why the Correct Answer is Right**
The treatment of multiple carboxylase deficiency involves administering a cofactor called biotin, which is essential for the proper functioning of the affected enzymes. Biotin acts as a coenzyme for various carboxylase enzymes, facilitating the transfer of carbon dioxide and the subsequent carboxylation reactions. Supplementing with biotin helps to alleviate the symptoms of the disorder by providing the necessary cofactor for enzyme activity.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not applicable, as there is no established treatment involving vitamin B12 for multiple carboxylase deficiency.
* **Option B:** Not correct, as carnitine supplementation is not a recognized treatment for multiple carboxylase deficiency.
* **Option C:** Not correct, as riboflavin supplementation is not a standard treatment for this condition.

**Clinical Pearl / High-Yield Fact**
Biotin supplementation is a crucial aspect of managing multiple carboxylase deficiency, and patients often require lifelong supplementation to maintain enzyme activity and prevent the accumulation of toxic metabolites.

**Correct Answer: B. Biotin**