Casal's necklace is a dermal condition that manifests as a rash around the neck. It is a clinical sign associated with pellagra, which is a disease caused by the deficiency of Niacin (Vitamin B3).
Let's examine each option to understand why Niacin is the correct answer:
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Cyanocobalamin: This is Vitamin B12. Its deficiency typically causes symptoms related to the nervous system and anemia (such as pernicious anemia) but not pellagra or Casal's necklace.
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Pyridoxine: This is Vitamin B6. Deficiency in pyridoxine can lead to symptoms such as anemia, depression, or weakened immune function, but not Casal's necklace.
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Riboflavin: This is Vitamin B2. Deficiency can lead to conditions such as angular stomatitis or glossitis, but it is not linked with Casal's necklace.
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Niacin: Deficiency in Niacin leads to pellagra, which is characterized by the "three Ds" - dermatitis, diarrhea, and dementia. Casal's necklace is a specific type of dermatitis that appears around the neck and is indicative of this deficiency.
Pellagra and Casal's necklace are directly linked to a lack of Niacin. This dermal symptom occurs because lack of Niacin affects energy production and the skin's health, leading to specific dermal manifestations when exposed to sunlight.
Therefore, the correct answer is Niacin.