## **Core Concept**
The question tests knowledge of amino acid classification based on their side chain properties. Amino acids can be categorized into several groups, including those containing sulfur. Sulfur-containing amino acids play critical roles in various biological processes.

## **Why the Correct Answer is Right**
The correct answer, **C. Methionine and Cysteine**, is accurate because both methionine and cysteine are amino acids that contain sulfur in their side chains. Methionine contains a sulfur atom in the form of a thioether, while cysteine contains a thiol (sulfhydryl) group. These sulfur-containing amino acids are crucial for protein structure and function. Methionine is an essential amino acid, meaning it cannot be synthesized by the human body and must be obtained through the diet. Cysteine, on the other hand, can be synthesized from methionine.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent sulfur-containing amino acids.
- **Option B:** This option is incorrect as it likely includes amino acids that do not contain sulfur or inaccurately represents the sulfur-containing amino acids.
- **Option D:** This option is incorrect because it does not correctly identify methionine and cysteine as the sulfur-containing amino acids.

## **Clinical Pearl / High-Yield Fact**
A memorable point for exams is that methionine and cysteine are the two amino acids containing sulfur. Cysteine can form disulfide bonds, which are important for the stability and folding of many proteins. A classic clinical correlation is the role of N-acetylcysteine, a derivative of cysteine, as a mucolytic agent and in the treatment of acetaminophen overdose.

## **Correct Answer:** C. Methionine and Cysteine