## **Core Concept**
The question tests understanding of protein structure and the factors influencing ultraviolet (UV) absorbance at 280 nm. This wavelength is specifically absorbed by certain amino acids, leading to increased absorbance.

## **Why the Correct Answer is Right**
The correct answer, **Tryptophan**, contains an indole structure, which significantly absorbs UV light at 280 nm. This property is due to the presence of the tryptophan side chain, which has a large conjugated system, allowing it to absorb light in the UV range. When tryptophan replaces alanine (a non-absorbing amino acid) in a protein, the overall UV absorbance at 280 nm increases.

## **Why Each Wrong Option is Incorrect**
* **Option A:** While **Tyrosine** does absorb UV light, its absorbance peak is more at 274 nm, and it does not absorb as strongly as tryptophan at 280 nm. However, it still contributes to UV absorbance.
* **Option B:** **Cysteine** does not significantly absorb UV light at 280 nm. Its absorbance is more relevant at lower wavelengths.
* **Option D:** **Alanine** itself does not absorb UV light at 280 nm; it's used here as a baseline for replacement.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that tryptophan, along with tyrosine and to a lesser extent phenylalanine, contributes to a protein's UV absorbance at 280 nm. This property is often exploited in biochemical assays to quantify protein concentrations.

## **Correct Answer:** .