## **Core Concept**
The question tests knowledge on the detection methods for cocaine metabolites. Cocaine is a commonly abused substance, and its metabolites can be detected in various biological samples using different assays. Understanding the pharmacokinetics and detection methods of cocaine is crucial in clinical and forensic toxicology.

## **Why the Correct Answer is Right**
The correct answer involves understanding the primary metabolite of cocaine and the assay most likely to detect it. Cocaine is primarily metabolized into **benzoylecgonine**, which is then detected through immunoassay techniques. Immunoassays are widely used for initial screening due to their sensitivity and specificity for detecting drugs and their metabolites, including benzoylecgonine.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although chromatography-mass spectrometry (GC-MS or LC-MS) is highly specific and sensitive for detecting cocaine metabolites, it is typically used for confirmation rather than initial screening due to its complexity and cost.
- **Option B:** This option is incorrect as it does not directly relate to common methods for detecting cocaine metabolites. Without specifics, it's hard to assess, but generally, other types of assays might not be as directly relevant or commonly used for cocaine metabolite detection.
- **Option C:** This option is incorrect because, while it might represent another type of assay, it's not specified and generally not as commonly associated with cocaine metabolite detection as immunoassay.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **immunoassay (Option D)** is often the first-line method for detecting cocaine use due to its sensitivity for benzoylecgonine, the primary metabolite of cocaine. However, results are usually confirmed with more specific techniques like GC-MS due to the potential for false positives.

## **Correct Answer:** D. Immunoassay.