**Core Concept**
Kunkel's test is a forensic diagnostic procedure used to detect the presence of carbon monoxide (CO) in blood. It relies on the principle that CO binds to hemoglobin more strongly than oxygen, forming carboxyhemoglobin, which can be identified through chemical tests.

**Why the Correct Answer is Right**
In Kunkel's test, blood is mixed with potassium ferrocyanide and sodium hydroxide. If carbon monoxide is present, it reacts with hemoglobin to form carboxyhemoglobin, which, when treated with ferrocyanide, produces a characteristic brown precipitate. This reaction is specific to CO and confirms its presence in the blood, making it a key test in cases of suspected carbon monoxide poisoning.

**Why Each Wrong Option is Incorrect**
Option A: Lead is detected by the **lead test** (using sodium sulfide), not Kunkel’s test.
Option B: CuSO₄ is a reagent used in other tests, such as for copper or in urine analysis, not in Kunkel’s test.
Option D: Dhatura (a plant) contains tropane alkaloids and is detected by **alkaloid tests**, not Kunkel’s test.

**Clinical Pearl / High-Yield Fact**
Always remember: **Kunkel’s test detects carbon monoxide (CO) in blood** — a hallmark of poisoning, especially in closed environments like vehicles or poorly ventilated rooms. A positive test (brown precipitate) confirms CO exposure, which may be missed by clinical symptoms alone.

✓ Correct Answer: C. CO