## **Core Concept**
Semi-essential or conditionally essential amino acids are those that can be synthesized by the human body under normal conditions but may become essential in times of stress or when the body's demand for them exceeds its ability to produce them. These amino acids include arginine, tyrosine, cysteine, glutamine, proline, and glycine.

## **Why the Correct Answer is Right**
Tyrosine (Option C) is considered a semi-essential amino acid because it can be synthesized from the essential amino acid phenylalanine through the action of the enzyme phenylalanine hydroxylase. However, in certain conditions, such as phenylketonuria (PKU) where this conversion pathway is impaired, tyrosine becomes essential.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and does not provide any information to assess its correctness.
- **Option B:** This option is also blank and cannot be evaluated.
- **Option D:** This option is blank as well, making it impossible to determine its accuracy.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while amino acids are categorized into essential, semi-essential, and non-essential, the body's requirement for these amino acids can change based on physiological conditions. For example, arginine is considered semi-essential because, although it can be synthesized by the body, supplementation may be beneficial in certain conditions like severe illness or injury when the body's demand exceeds its production capacity.

## **Correct Answer:** C. Tyrosine.