**Core Concept**
Delayed bone marrow suppression leading to sustained neutropenia is a rare but serious side effect of certain medications that affect rapidly dividing cells. This phenomenon is characterized by a prolonged decrease in white blood cell count, particularly neutrophils, following the administration of a causative agent.

**Why the Correct Answer is Right**
The correct answer, **C. Leflunomide**, is an immunosuppressive disease-modifying antirheumatic drug (DMARD) used in the treatment of rheumatoid arthritis. Leflunomide works by inhibiting dihydroorotate dehydrogenase, an enzyme essential for pyrimidine synthesis, thereby suppressing T and B lymphocyte proliferation. The mechanism of delayed bone marrow suppression is thought to be related to the drug's effect on rapidly dividing hematopoietic stem cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not directly relate to the causation of delayed bone marrow suppression.
* **Option B:** This option is incorrect because, although it can cause myelosuppression, it is not typically associated with delayed bone marrow suppression.

**Clinical Pearl / High-Yield Fact**
It is essential to note that patients on leflunomide who develop delayed bone marrow suppression may require prolonged treatment with growth factors and supportive care to manage neutropenia.

**Correct Answer:** C. Leflunomide