**Core Concept**
The Marquis test is a chemical test used in forensic medicine to detect the presence of certain substances, specifically opioids. It involves the reaction of a sample with a reagent to produce a characteristic color change, which indicates the presence of the substance being tested.

**Why the Correct Answer is Right**
The Marquis test is specifically used to detect morphine and other opioids. The test involves adding a few drops of Marquis reagent to a sample, which contains a mixture of morphine and other substances. The Marquis reagent reacts with the morphine to produce a purple color, indicating the presence of the opioid. This reaction is due to the reduction of the Marquis reagent by the morphine, resulting in the formation of a purple-colored compound. The test is widely used in forensic medicine to detect morphine and other opioids in cases of suspected poisoning or overdose.

**Why Each Wrong Option is Incorrect**
**Option A:** Mercury poisoning is not detected using the Marquis test. Mercury poisoning is typically detected using other tests, such as atomic absorption spectroscopy or inductively coupled plasma mass spectrometry.

**Option B:** Arsenic poisoning is not detected using the Marquis test. Arsenic poisoning is typically detected using other tests, such as atomic absorption spectroscopy or inductively coupled plasma mass spectrometry.

**Option D:** Cyanide poisoning is not detected using the Marquis test. Cyanide poisoning is typically detected using other tests, such as the silver nitrate test or the ferric chloride test.

**Clinical Pearl / High-Yield Fact**
The Marquis test is a simple and rapid test that can be performed in the field or in a laboratory setting. It is an important tool in forensic medicine for detecting morphine and other opioids, and can help investigators determine the cause of death in cases of suspected poisoning or overdose.

**✓ Correct Answer: C. Morphine poisoning**