**Core Concept**
Metallothionein is a low-molecular-weight, cysteine-rich protein that plays a crucial role in the regulation of heavy metal homeostasis and detoxification. It is induced by exposure to various heavy metals, such as cadmium, mercury, and lead.

**Why the Correct Answer is Right**
The induction of metallothionein is mediated by the activation of the metal response element (MRE) in the metallothionein gene promoter. This activation is triggered by the binding of metal transcription factor-1 (MTF-1) to the MRE, which is in turn activated by the presence of heavy metals. The heavy metals bind to the thiol groups of metallothionein, inducing its synthesis and promoting the detoxification of the metal.

**Why Each Wrong Option is Incorrect**
**Option A:** Zinc is an essential mineral, but it does not induce the synthesis of metallothionein. Instead, it is a cofactor for many enzymes that require metallothionein for their activity.
**Option B:** Copper is an essential mineral that plays a crucial role in various biological processes, but it does not induce the synthesis of metallothionein. Copper toxicity is actually mitigated by metallothionein, which binds to copper and prevents its accumulation.
**Option C:** Iron is an essential mineral that is critical for oxygen transport and electron transfer, but it does not induce the synthesis of metallothionein. Iron overload can actually lead to tissue damage and disease, but metallothionein does not play a direct role in its regulation.

**Clinical Pearl / High-Yield Fact**
Metallothionein is induced by exposure to heavy metals, but it also has a role in the regulation of zinc homeostasis. Zinc is a cofactor for many enzymes that require metallothionein for their activity, and zinc deficiency can lead to impaired metallothionein function.

**Correct Answer:** C. Cadmium. Cadmium is a toxic heavy metal that induces the synthesis of metallothionein, allowing the body to detoxify and eliminate it.