## Core Concept
The question tests knowledge of vitamin deficiencies and their associated clinical manifestations, specifically glossitis (inflammation of the tongue) and cheilosis (inflammation of the lips).

## Why the Correct Answer is Right
Riboflavin, also known as Vitamin B2, plays a crucial role in energy production and can cause mucocutaneous lesions when deficient. Glossitis and cheilosis are classic signs of riboflavin deficiency, along with seborrheic dermatitis and corneal vascularization. Riboflavin is essential for the synthesis of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which are vital for various metabolic processes.

## Why Each Wrong Option is Incorrect
* **Option A:** Vitamin B1 (Thiamine) deficiency primarily causes beriberi and Wernicke-Korsakoff syndrome, characterized by cardiovascular and neurological symptoms, not typically glossitis and cheilosis.
* **Option C:** Vitamin B6 (Pyridoxine) deficiency can cause dermatitis, depression, confusion, and peripheral neuropathy but is less commonly associated with glossitis and cheilosis compared to riboflavin.
* **Option D:** Vitamin B12 deficiency is known for causing megaloblastic anemia, neuropathy, and specific oral manifestations like atrophic glossitis but is not the most direct cause of both glossitis and cheilosis together.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that riboflavin deficiency often presents with ocular, oral, and genital lesions. A simple mnemonic to remember is that riboflavin deficiency causes "4 Ds": Diarrhea, Dermatitis, cheilosis (or Dryness of mucous membranes), and glossitis (or Dyspepsia).

## Correct Answer: B. Riboflavin.