## Core Concept
Metallothioneins (MTs) are a family of low-molecular-weight, cysteine-rich proteins that play a significant role in maintaining metal homeostasis and protecting against metal toxicity in cells. They are characterized by their ability to bind metals through the thiol group of its cysteine residues.

## Why the Correct Answer is Right
The correct answer, **Mercury (Hg2+)**, along with other heavy metals like lead, cadmium, and arsenic, can be sequestered by metallothioneins. These proteins have a high affinity for heavy metals, which allows them to protect cells from the toxic effects of these metals by forming stable complexes. The thiol groups of the cysteine residues in metallothioneins are crucial for this metal-binding capacity.

## Why Each Wrong Option is Incorrect
- **Option A: Calcium (Ca2+)** - While essential for numerous cellular functions, calcium ions are not typically sequestered by metallothioneins. Calcium homeostasis is maintained by other proteins and mechanisms.
- **Option B: Potassium (K+)** - This is an alkali metal crucial for maintaining the resting membrane potential in cells. It is not bound by metallothioneins.
- **Option D: Zinc (Zn2+)** - Although metallothioneins do bind zinc, the context of the question emphasizes the sequestration of "harmful" metal ions. Zinc is an essential trace element necessary for numerous biological functions.

## Clinical Pearl / High-Yield Fact
A key point to remember is that metallothioneins are involved in protecting against heavy metal toxicity and are also implicated in zinc and copper homeostasis. Their role in heavy metal detoxification makes them significant in the context of environmental and occupational health.

## Correct Answer Line
**Correct Answer: C. Mercury (Hg2+).**