**Core Concept**
The patient's symptoms of sun-sensitive dermatitis, diarrhea, and dementia are indicative of a disorder related to a specific vitamin deficiency. This condition is characterized by impaired metabolism of fatty acids and other lipids, leading to various systemic manifestations.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with a deficiency in **Vitamin B7 (Biotin)**. Biotin is a water-soluble vitamin that plays a crucial role in carboxylation reactions, particularly in the metabolism of fatty acids and amino acids. A deficiency in biotin can lead to impaired fatty acid synthesis, resulting in the accumulation of mevalonate, which is responsible for the skin symptoms. Additionally, biotin deficiency can affect the nervous system, leading to symptoms such as dementia and neurological dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a deficiency in Vitamin B6 (Pyridoxine) primarily affects neurotransmitter synthesis and can lead to symptoms such as peripheral neuropathy, but not sun-sensitive dermatitis.
**Option B:** This option is incorrect because a deficiency in Vitamin B2 (Riboflavin) affects energy production and can lead to symptoms such as cheilosis and glossitis, but not dementia.
**Option C:** This option is incorrect because a deficiency in Vitamin B3 (Niacin) affects energy production and can lead to symptoms such as pellagra, but not sun-sensitive dermatitis.

**Clinical Pearl / High-Yield Fact**
Biotin deficiency is often seen in individuals with malabsorption syndromes, such as Crohn's disease, or those taking long-term antibiotics, which can disrupt the gut microbiome and impair biotin absorption.

**Correct Answer:** B.