**Core Concept**
Tryptophan catabolism is a complex biochemical process that involves the breakdown of the essential amino acid tryptophan into various metabolites. This process is initiated by the enzyme tryptophan 2,3-dioxygenase (TDO) or indoleamine 2,3-dioxygenase (IDO), which catalyzes the first step in tryptophan degradation.

**Why the Correct Answer is Right**
The first product of tryptophan catabolism is indeed kynurenine, which is formed through the oxidative cleavage of tryptophan by TDO or IDO. This reaction involves the incorporation of oxygen into the tryptophan molecule, resulting in the formation of N-formylkynurenine, which is then converted to kynurenine. Kynurenine is a key intermediate in the kynurenine pathway, which is responsible for the majority of tryptophan catabolism in mammals.

**Why Each Wrong Option is Incorrect**
**Option B:** Xantheurenic acid is a metabolite of tryptophan, but it is not the first product of tryptophan catabolism. It is formed through the kynurenine pathway, specifically through the action of kynureninase.
**Option C:** Bradykinin is a peptide involved in the regulation of blood pressure and inflammation, but it is not a product of tryptophan catabolism. Bradykinin is formed through the action of kallikrein on kininogen.
**Option D:** Melatonin is a hormone involved in the regulation of sleep-wake cycles, but it is not a product of tryptophan catabolism. Melatonin is formed through the action of the enzyme aralkylamine N-acetyltransferase on N-acetylserotonin.

**Clinical Pearl / High-Yield Fact**
The kynurenine pathway is a major route of tryptophan catabolism in mammals, and it is involved in the regulation of various physiological processes, including inflammation and immune response. Additionally, the kynurenine pathway is a target for the development of novel therapeutic agents, including those for the treatment of neurodegenerative diseases and cancer.

**✓ Correct Answer: A. Kynurenine**