A 14-year-old girl with decreased tryptophan levels is likely experiencing an impairment in the production of which vitamin?

The correct answer focuses on niacin, which is also known as vitamin B3. Tryptophan is an essential amino acid that serves as a precursor for the synthesis of niacin in the body. Under normal physiological conditions, the body can convert tryptophan into niacin through a series of enzymatic reactions. However, if there are decreased levels of tryptophan, the body's ability to produce niacin may be compromised. This could lead to symptoms associated with niacin deficiency, such as fatigue, skin problems, digestive issues, and neurological symptoms.

In contrast, the other vitamins listed do not have this direct dependency on tryptophan. Vitamin C, Vitamin B2 (riboflavin), and Vitamin B5 (pantothenic acid) are synthesized and obtained through different dietary sources and metabolic pathways, making them unaffected by tryptophan levels. Understanding the biochemical roles of these vitamins helps clarify why niacin is the correct choice in the context of decreased tryptophan levels.