Step 1: Understanding the Question: The question specifically asks us to confidently link a recognized toxic heavy metal to the occurrence of "Itai-Itai" disease in humans.
Step 2: Key Concept: Unregulated industrial pollution unfortunately often leads to the massive bioaccumulation of specific heavy metals, which then trigger explicitly recognized and named medical syndromes in human populations.
Step 3: Detailed Explanation:
The tragic medical term "Itai-Itai" directly translates to "it hurts, it hurts" or "ouch-ouch" in Japanese, graphically characterizing the severe, agonizing joint and spine bone pain continuously suffered by the victims.
The disease painfully emerged in the Jinzu River basin in Toyama Prefecture, Japan, during the early \(20\text{th}\) century.
Irresponsible mining companies routinely dumped highly acidic wastewater heavily laden with toxic heavy metals directly into the local river system.
This contaminated river water was then widely used by farmers to artificially irrigate vast rice paddy fields.
Consequently, Cadmium heavily bioaccumulated in the growing rice crops over years.
Chronic consumption of this deeply cadmium-tainted rice tragically caused severe irreversible renal failure and massive osteomalacia (severe softening of the bones), directly leading to the extreme pain and multiple bone fractures defining the disease.
In stark contrast, Mercury poisoning causes Minamata disease, and long-term Arsenic exposure causes Arsenicosis (commonly presenting as black foot disease).
Step 4: Final Answer: Cadmium is unambiguously the heavy metal strictly responsible for causing Itai-Itai disease.