**Core Concept**
Lithium toxicity occurs when plasma lithium concentrations exceed the therapeutic range, causing adverse effects on the central nervous system and kidneys. This is due to lithium's narrow therapeutic index and the risk of accumulation in the body.

**Why the Correct Answer is Right**
The toxic dose of lithium is typically considered to be above 1.5 mmol/L, with severe toxicity occurring at concentrations above 2.5 mmol/L. At these levels, lithium can cause neurological symptoms such as confusion, seizures, and coma, as well as renal impairment and electrolyte imbalances. Lithium toxicity is often precipitated by factors such as dehydration, renal impairment, or the concurrent use of other medications that affect lithium levels.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the toxic dose of lithium is not typically considered to be below 1.2 mmol/L. While lithium toxicity can occur at lower concentrations, this value is generally considered to be within the therapeutic range.
**Option B:** This option is incorrect because lithium toxicity is not typically caused by plasma concentrations below 1.2 mmol/L. While lithium toxicity can occur at lower concentrations, this value is generally considered to be within the therapeutic range.
**Option C:** This option is incorrect because the toxic dose of lithium is not typically considered to be above 3.5 mmol/L. While lithium toxicity can occur at higher concentrations, this value is generally considered to be above the typical toxic range.

**Clinical Pearl / High-Yield Fact**
Lithium toxicity can be precipitated by factors such as dehydration, renal impairment, or the concurrent use of other medications that affect lithium levels. It is essential to monitor plasma lithium concentrations regularly in patients taking lithium and to adjust the dose accordingly to minimize the risk of toxicity.

**Correct Answer: D. 2.5 mmol/L**