**Core Concept**
Tryptophan catabolism involves the breakdown of the essential amino acid tryptophan into various metabolites. This process is crucial for nitrogen metabolism and the production of neurotransmitters such as serotonin and melatonin. The initial step in tryptophan catabolism is mediated by the enzyme tryptophan pyrrolase (also known as tryptophan 2,3-dioxygenase).

**Why the Correct Answer is Right**
The first product of tryptophan catabolism is N-formylkynurenine, which is formed by the action of tryptophan pyrrolase on tryptophan. This enzyme catalyzes the oxidative cleavage of the indole ring, resulting in the formation of N-formylkynurenine and formic acid. N-formylkynurenine is then further metabolized to kynurenine, which is the next step in tryptophan catabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not the first product of tryptophan catabolism.
**Option B:** Incorrect because it is not directly involved in the initial step of tryptophan catabolism.
**Option C:** Incorrect because it is not a correct representation of the first product of tryptophan catabolism.

**Clinical Pearl / High-Yield Fact**
Tryptophan pyrrolase is inducible by glucocorticoids, which can lead to increased tryptophan catabolism and decreased tryptophan availability for protein synthesis.

**Correct Answer:** C. N-formylkynurenine