**Core Concept**
Strychnine is a highly toxic, colorless, crystalline alkaloid used as a rodenticide and in the past as an ingredient in some pharmaceuticals and medicinal products. Its mechanism of action involves the blockade of inhibitory neurotransmission in the central nervous system, leading to hyperexcitability and muscle convulsions.

**Why the Correct Answer is Right**
The fatal dose of strychnine is estimated to be between 60-100 milligrams (mg) when ingested orally. This dose can cause severe muscle convulsions, respiratory failure, and eventually death. Strychnine acts by blocking the action of the inhibitory neurotransmitter glycine at the strychnine-sensitive glycine receptors in the spinal cord and brainstem, leading to an uncontrolled increase in muscle tone and convulsive activity. The onset of symptoms typically occurs within 15-30 minutes of ingestion.

**Why Each Wrong Option is Incorrect**
**Option A:** 10-15 mg is considered a sublethal dose, which may result in mild symptoms such as muscle twitching and weakness, but is unlikely to cause death.

**Option B:** 20-40 mg is also considered sublethal and may result in more severe symptoms such as muscle convulsions and respiratory distress, but is still below the fatal dose.

**Option D:** 100-120 mg is above the estimated fatal dose, but the exact range is not well-defined, and doses above 100 mg are generally considered to be highly toxic and potentially fatal.

**Clinical Pearl / High-Yield Fact**
It is essential to note that strychnine poisoning can be challenging to diagnose, and symptoms may resemble those of other conditions such as tetanus or seizures. A high index of suspicion and prompt medical attention are crucial in managing strychnine poisoning.

**✓ Correct Answer: C. 60-100 mg**