**Core Concept**
The catabolism of tryptophan is a complex process involving multiple enzymes and steps. Tryptophan is an essential amino acid that can be broken down into various products through different pathways. The initial steps of tryptophan catabolism involve the conversion of tryptophan into kynurenine, which is then further metabolized.

**Why the Correct Answer is Right**
The first product of tryptophan catabolism is indeed kynurenine. This occurs through the action of the enzyme tryptophan 2,3-dioxygenase (TDO) or indoleamine 2,3-dioxygenase (IDO), which catalyze the oxidation of tryptophan to form N-formylkynurenine. N-formylkynurenine is then converted into kynurenine through the action of the enzyme kynurenine formamidase.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not represent a known product of tryptophan catabolism.
* **Option B:** This option is incorrect because it is a downstream product of tryptophan catabolism, not the initial product.
* **Option D:** This option is incorrect because it is not a known product of tryptophan catabolism.

**Clinical Pearl / High-Yield Fact**
Kynurenine is a key intermediate in the catabolism of tryptophan and is involved in the production of various neuroactive compounds, including quinolinic acid and kynurenic acid. These compounds have been implicated in the pathophysiology of various neurological disorders, including Alzheimer's disease and schizophrenia.

**Correct Answer:** C. Kynurenine