Final answer:
Hartnup disease and Vitamin B3 deficiency present with similar symptoms, including pellegra-like dermatitis and mental changes, but have different causes; the former is genetic while the latter is typically dietary.
Step-by-step explanation:
The question addresses the similarities and differences between Hartnup disease and Vitamin B3 deficiency. Hartnup disease is a rare metabolic disorder characterized by defective transport of certain amino acids including tryptophan, which can result in a secondary niacin (Vitamin B3) deficiency because tryptophan is needed to synthesize niacin. As a result, individuals with Hartnup disease may exhibit symptoms similar to pellagra, which is caused by niacin deficiency itself. These symptoms include a dermatitis known as pellegra-rash, diarrhea, dementia, and if left untreated, the condition may lead to death.
Therefore, the correct answer to the question is (a) Similar symptoms but different causes. Hartnup disease is caused by a genetic defect affecting amino acid transport, and niacin deficiency is usually due to poor diet. Despite their different causes, they can share similar clinical presentations.