**Core Concept**
Synesthesia is a neurological condition where one sense is simultaneously perceived as if by one or more additional senses. In the context of drug abuse, certain substances can induce a temporary state of synesthesia, often referred to as a "cross-over" of sensory perceptions.

**Why the Correct Answer is Right**
LSD (Lysergic acid diethylamide) is a potent hallucinogenic drug known to alter perception, thought, and mood. It works by binding to serotonin receptors in the brain, particularly the 5-HT2A receptor subtype. This binding causes an increase in the release of serotonin, leading to a cascade of neurochemical events that result in the characteristic altered states of consciousness, including synesthesia. The specific mechanism by which LSD induces synesthesia is not fully understood, but it is thought to involve the activation of neurons in the visual and auditory cortices, leading to the fusion of sensory information.

**Why Each Wrong Option is Incorrect**
**Option A:** Cocaine is a powerful stimulant that primarily affects the dopamine system in the brain, leading to increased alertness, energy, and euphoria. While it can cause psychosis and paranoia in high doses, it is not typically associated with synesthesia.

**Option C:** Marijuana, also known as cannabis, is a psychoactive substance that affects the brain's cannabinoid receptors. Its primary effects include altered perception, relaxation, and altered mood. However, it is not typically associated with synesthesia.

**Option D:** PCP (Phencyclidine) is a dissociative anesthetic that can induce a range of effects, including hallucinations, disorientation, and detachment from reality. While it can cause altered sensory experiences, it is not typically associated with the specific phenomenon of synesthesia.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the effects of LSD can be unpredictable and may vary widely between individuals. Additionally, the use of LSD can have long-term psychological consequences, including the development of anxiety disorders and psychosis.

**✓ Correct Answer: B. LSD**