**Core Concept**
Niacin, also known as vitamin B3 or nicotinic acid, is an essential nutrient that plays a critical role in various metabolic processes. It serves as a precursor to NAD+ and NADP+, which are essential cofactors in redox reactions and energy metabolism.

**Why the Correct Answer is Right**
Tryptophan is an amino acid that can be converted into niacin through a series of enzymatic reactions. This process involves the kynurenine pathway, where tryptophan is first converted into kynurenine by the enzyme kynureninase. Kynurenine is then converted into 3-hydroxykynurenine, which is eventually converted into niacin. This pathway is particularly important in organisms that cannot synthesize niacin de novo, including humans.

**Why Each Wrong Option is Incorrect**
* **Option A:** Phenylalanine is an essential amino acid that cannot be converted into niacin. It is involved in the synthesis of tyrosine, but not niacin.
* **Option C:** Tyrosine is an amino acid that can be synthesized from phenylalanine, but it is not a precursor to niacin.
* **Option D:** Methionine is a non-essential amino acid that can be synthesized from homocysteine, but it is not involved in the synthesis of niacin.

**Clinical Pearl / High-Yield Fact**
The kynurenine pathway is a critical route for tryptophan metabolism, and alterations in this pathway have been implicated in various diseases, including neurological disorders and cancer. It is also worth noting that niacin deficiency can lead to pellagra, a condition characterized by dermatitis, diarrhea, dementia, and death.

**✓ Correct Answer: B. Tryptophan**