The disease caused by deficiency of protein in children is

Kwashiorkor is the classic protein deficiency disease in children, usually seen when a child is weaned early onto a starchy, protein-poor diet. Calorie intake may look reasonable, which is why the child appears swollen with oedema of the feet and abdomen rather than wasted, often with an enlarged fatty liver and changes in hair and skin.

Why the others are wrong:

• Pellagra is caused by deficiency of niacin (vitamin B3), and presents with dermatitis, diarrhoea and dementia.
• Beri beri is thiamine (vitamin B1) deficiency, affecting nerves and heart, common where polished rice is the staple.
• Rickets is a bone disorder from vitamin D deficiency and poor calcification, not protein.

Core concept: protein-energy malnutrition. Kwashiorkor (protein deficiency with relatively adequate energy) is contrasted with marasmus (overall energy deficiency). This sits under health and nutrition in GS-II, and is a useful Mains angle on child malnutrition and POSHAN.

Trap: candidates treat all four as "deficiency diseases" and pick a vitamin deficiency by reflex. Remember that kwashiorkor is the protein one; the other three are vitamin deficiencies.