**Core Concept**
Busulfan is an alkylating agent used in the treatment of chronic myeloid leukemia. Its primary mechanism of action involves the formation of cross-links between DNA strands, thereby inhibiting DNA replication and transcription. Busulfan's toxicity profile is a critical consideration in its clinical use.

**Why the Correct Answer is Right**
The primary toxicity of busulfan is bone marrow suppression, which is a result of its alkylating effects on the DNA of rapidly dividing cells. Busulfan's ability to cross-link DNA strands in the bone marrow leads to a decrease in the production of blood cells, resulting in anemia, neutropenia, and thrombocytopenia. This effect is a direct consequence of busulfan's mechanism of action, which targets the DNA of hematopoietic stem cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** While busulfan can cause gastrointestinal toxicity, it is not the primary toxicity associated with this medication.
* **Option B:** Hepatotoxicity is a potential side effect of busulfan, but it is not the primary toxicity.
* **Option C:** Pulmonary toxicity is not a well-documented primary toxicity of busulfan.

**Clinical Pearl / High-Yield Fact**
Busulfan's primary toxicity is a critical consideration in the management of patients with chronic myeloid leukemia. The use of busulfan in combination with other chemotherapeutic agents, such as cyclophosphamide and total body irradiation, is a common approach in conditioning regimens for hematopoietic stem cell transplantation.

**Correct Answer:** D. Bone marrow suppression