**Core Concept**
Nonsteroidal immunosuppressive drugs like Methotrexate (Mtx) can cause pulmonary toxicity, manifesting as respiratory symptoms in some patients. This is a rare but serious side effect of long-term Mtx therapy.

**Why the Correct Answer is Right**
Mtx-induced lung injury is thought to occur through the generation of reactive oxygen species and the activation of immune cells, leading to inflammation and damage to lung tissue. The exact mechanism is not fully understood, but it is believed to involve the release of cytokines and the activation of various signaling pathways. In susceptible individuals, this can result in symptoms such as shortness of breath, cough, and chest pain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a known side effect of Mtx. Gastrointestinal toxicity is a more common side effect of Mtx, but it does not typically present with respiratory symptoms.

**Option B:** While interstitial lung disease is a known side effect of certain medications, it is not a direct consequence of Mtx therapy.

**Option C:** This option is not a recognized side effect of Mtx. The medication can cause various other side effects, but respiratory symptoms are not typically associated with this condition.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor patients on long-term Mtx therapy for signs of pulmonary toxicity, including respiratory symptoms, and to consider lung function tests and imaging studies if these symptoms develop.

**Correct Answer:** C.