**Core Concept**
Amino acid tests are crucial in clinical settings to diagnose and monitor various conditions, including metabolic disorders. These tests assess the levels of amino acids in the blood, which can help identify underlying pathologies.

**Why the Correct Answer is Right**
Amino acid tests typically include methods such as the sulfosalicylic acid (SSA) test, which detects the presence of albumin and thus indirectly assesses the level of albumin-bound amino acids. The biuret test is another method used to quantify albumin. However, the **phenylketonuria (PKU) test** specifically detects phenylalanine levels, which is a specific amino acid disorder. The **Sulphur test** is used to detect the presence of sulphur-containing amino acids.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a correct option as it was not provided in the question.

**Option B:** This is not a correct option as it was not provided in the question.

**Option C:** This is not a correct option as it was not provided in the question.

**Clinical Pearl / High-Yield Fact**
It's essential to note that various tests have different applications and limitations. The choice of test depends on the specific clinical scenario and the suspected underlying condition.

**Correct Answer:** D.