Which vitamin deficiency can lead to conditions affecting the skin and mucous membranes?

The correct choice is Nicotinic acid, also known as niacin (Vitamin B3), as its deficiency is linked to conditions that significantly affect the skin and mucous membranes. Specific conditions associated with niacin deficiency include pellagra, which is characterized by three main symptoms known as the "three D's": dermatitis, diarrhea, and dementia. The skin changes can manifest as a scaly, red rash, particularly in areas exposed to sunlight, highlighting the relationship between this vitamin and skin health.

Thiamine (Vitamin B1), Vitamin B2 (riboflavin), and Pyridoxine (Vitamin B6) are vital for various body functions, including energy metabolism and neurotransmitter synthesis; however, their deficiencies do not typically result in the same skin and mucous membrane issues that occur with niacin deficiency. This makes Nicotinic acid the key vitamin in this context for maintaining the health of the skin and mucous membranes.