**Core Concept**
Magnan's symptom refers to a specific type of tactile hallucination where patients feel as if they have insects crawling under their skin. This phenomenon is associated with the use of certain substances that affect the central nervous system.

**Why the Correct Answer is Right**
Magnan's symptom is classically seen in individuals who have used cocaine, particularly in cases of cocaine-induced psychosis. The mechanism underlying this symptom is thought to be related to the stimulation of dopamine receptors in the brain, leading to altered sensory perception and hallucinations. The tactile hallucinations associated with Magnan's symptom are often described as feeling like insects crawling under the skin.

**Why Each Wrong Option is Incorrect**
**Option A:** Datura is a plant species that contains scopolamine and atropine, which can cause anticholinergic symptoms such as hallucinations, but it is not typically associated with Magnan's symptom.
**Option C:** Opium is an opioid that can cause a range of psychological and physical symptoms, including euphoria and sedation, but it is not commonly linked with tactile hallucinations like Magnan's symptom.
**Option D:** Cannabis is a psychoactive substance that can cause a variety of effects, including altered perception and mood changes, but it is not typically associated with Magnan's symptom.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Magnan's symptom is a classic example of a substance-induced hallucination and can be a key feature in the diagnosis of cocaine-induced psychosis. This symptom can be a critical differentiator in forensic medicine, particularly in cases where patients may be presenting with altered mental states.

**Correct Answer:**
✓ Correct Answer: B. Cocaine