**Core Concept**
The underlying principle being tested is the metabolism of dietary tryptophan. **Tryptophan** is an essential amino acid that can be metabolized through various pathways, one of which leads to the production of indican.

**Why the Correct Answer is Right**
Indican is a metabolite of tryptophan, specifically produced when tryptophan is broken down by the gut flora. This process involves the conversion of tryptophan into **indole**, which is then further metabolized into indican. The correct answer choice should reflect the relationship between tryptophan metabolism and indican production.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not directly relate to the metabolism of tryptophan.
**Option B:** Incorrect as it is not a known precursor to indican.
**Option C:** This choice is incorrect because, although related to tryptophan, it is not the direct precursor to indican.

**Clinical Pearl / High-Yield Fact**
Remember that indican is also known as **indoxyl sulfate** in its conjugated form, which is excreted by the kidneys. Elevated levels of indican can indicate **gut dysbiosis** or **renal impairment**.

**Correct Answer:** D. Tryptophan