**Core Concept**
Inhalation of mercury vapour is a common route of exposure, which can lead to systemic toxicity due to the lipophilicity of the inorganic mercury ion (Hg2+). The body's ability to retain inhaled mercury vapour is influenced by several factors, including the solubility of mercury in blood and tissues, as well as the efficiency of the body's elimination mechanisms.

**Why the Correct Answer is Right**
About 80-90% of inhaled mercury vapour is retained in the body. This is because mercury vapour is highly soluble in blood and tissues, allowing it to easily cross biological membranes and distribute to various organs. Once inside the body, mercury vapour is rapidly oxidized to the more toxic inorganic mercury ion (Hg2+), which can then bind to sulfhydryl groups in proteins and enzymes, leading to cellular dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the actual retention fraction is higher than 10-20%. Mercury vapour is highly soluble in blood and tissues, allowing for efficient retention.
**Option B:** This option is incorrect because the actual retention fraction is higher than 50-60%. While some mercury vapour is exhaled or eliminated via other routes, the majority is retained in the body.
**Option C:** This option is incorrect because the actual retention fraction is not significantly lower than 80-90%. Mercury vapour is highly soluble and lipophilic, making it difficult to eliminate from the body.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the Hg2+ ion is a potent inhibitor of various enzymes, including glyoxalase I, which is involved in the detoxification of mercury. This can lead to accumulation of toxic metabolites and exacerbate mercury toxicity.

**Correct Answer:** C. 80-90%