**Core Concept**
Methotrexate toxicity is a clinical scenario where high doses or accumulation of methotrexate lead to folate antagonist effects, causing cellular and tissue damage. This is due to its inhibition of dihydrofolate reductase (DHFR) enzyme, which is crucial for DNA synthesis and repair. Methotrexate toxicity can manifest as gastrointestinal, hematological, and renal complications.

**Why the Correct Answer is Right**
The treatment of methotrexate toxicity involves replenishing folate stores and antagonizing the effects of methotrexate. **Leucovorin (folinic acid)** is a rescue agent that bypasses the inhibition of DHFR enzyme by methotrexate, allowing for the resumption of DNA synthesis and repair. Leucovorin is administered to rapidly reverse methotrexate toxicity, especially in cases of high-dose methotrexate therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **Activated charcoal** is not effective in treating methotrexate toxicity. Methotrexate is highly protein-bound, making it difficult for activated charcoal to bind and remove it from the body.

**Option B:** This option is incorrect because **Penicillamine** is used to treat heavy metal poisoning, not methotrexate toxicity. Penicillamine works by forming a stable complex with heavy metals, which can then be excreted from the body.

**Option C:** This option is incorrect because **Sulfasalazine** is an anti-inflammatory medication used to treat conditions like rheumatoid arthritis and ulcerative colitis. It is not used to treat methotrexate toxicity.

**Clinical Pearl / High-Yield Fact**
Remember that the timing and dose of leucovorin administration are critical in treating methotrexate toxicity. The American Society of Clinical Oncology (ASCO) recommends that leucovorin be administered within 24 hours of methotrexate exposure to minimize toxicity.

**Correct Answer: C. Leucovorin (folinic acid)**