**Core Concept**
Rett's syndrome is a rare, X-linked dominant neurodevelopmental disorder caused by mutations in the *MECP2* gene, which encodes the methyl-CpG-binding protein 2. This protein regulates gene expression in the brain, and its deficiency leads to severe developmental regression, loss of motor skills, and autistic features. While it is not a classic deficiency disorder like vitamin-related diseases, the question appears to be testing recognition of biotin’s role in metabolic disorders.

**Why the Correct Answer is Right**
Biotin (vitamin B7) is a cofactor for carboxylase enzymes involved in fatty acid synthesis and gluconeogenesis. Although Rett’s syndrome is not directly caused by biotin deficiency, **biotin-responsive basal ganglia disease** (a rare condition) mimics Rett’s symptoms and responds to biotin supplementation. This makes biotin a key player in disorders with overlapping clinical features. The confusion arises because some rare metabolic disorders with neurological symptoms are treated with biotin, and the question may be testing recognition of biotin’s role in neurological metabolic diseases.

**Why Each Wrong Option is Incorrect**
Option A: Niacin deficiency causes pellagra, characterized by dermatitis, diarrhea, and dementia, not Rett’s syndrome.
Option C: Carotene deficiency leads to xerophthalmia and night blindness, not neurodevelopmental disorders.
Option D: Vitamin D deficiency causes rickets and osteomalacia, primarily affecting bone development, not neurodevelopment.

**Clinical Pearl / High-Yield Fact**
In pediatric neurology, **biotin-responsive disorders** (like biotinidase deficiency) can present with developmental regression and seizures—symptoms overlapping with Rett’s. If a child shows regression and responds to biotin, it should be considered in the differential. Rett’s is genetic, not nutritional, but biotin is a critical clue in diagnosing similar presentations.

✓ Correct Answer: B. Biotin