Pellagra is caused by deficiency of
A) $B_3$
B) $B_5$
C) $B_6$
D) $B_2$

Hint:Pellagra is a skin disease which is caused by deficiency of vitamin B and its symptoms include dementia, sores in the mouth, diarrhea, dermatitis and inflamed skin. If the condition is not treated, then the individual may even die.

Complete answer:
Vitamins are organic micronutrients. They are required by the body in small quantities, but are very essential for our metabolism. A deficiency in any of the vitamins can result in one or more disorders.
There are two classes of vitamins: fat soluble and water soluble. The water soluble are further divided into the Vitamin C and the B complex vitamins.

Vitamin $B_3$ is also none as nicotinic acid and niacin. It is an important part of our metabolic cycles and is required for the health of our nervous system, G I tract, and skin. The deficiency of $B_3$ results in disorder characterised by diarrhoea, dementia, dermatitis, and death. This disease is called pellagra and often occurs due to diet deficient in leafy green vegetables, meats, and eggs. Option A is the correct answer.

Vitamin $B_5$ is also called pantothenic acid and is again important for metabolism and energy generation. Commonly found in most foods, the deficiency disease is paraesthesia. The symptoms include tickling and prickling sensations. Option B is not correct.

$B_6$ or pyridoxine is an important part of the process of blood cell production. Fund in meats, vegetables, and fruits, a deficiency of this vitamin can result in peripheral neuropathy and a type of anaemia. Option C is incorrect.

Lastly, vitamin $B_2$ or riboflavin is important for vision. It may help prevent the formation of cataracts. Deficiency symptoms of $B_2$ include cheilosis, hair loss, and angular stomatitis. Option D is incorrect.

Hence the correct answer is option ‘A’.

Note: While the water soluble vitamins have deficiency disorder, they rarely have problems with excess intake of the vitamin. This is because excess B and C vitamins are excreted in urine. Fat soluble vitamins are, however, stored in the body and have disorders connected to excess of Vitamin A, D, E, and K.