**Core Concept**
Myelophthisic anemia is a type of anemia characterized by the replacement of bone marrow by a fibrotic or neoplastic process, leading to a decrease in the production of normal hematopoietic cells. This results in a normocytic, normochromic anemia with a characteristic "pelger-huet" anomaly in neutrophils.

**Why the Correct Answer is Right**
The most common cause of myelophthisic anemia is **multiple myeloma**. In multiple myeloma, the proliferation of malignant plasma cells in the bone marrow leads to fibrosis and a decrease in the production of normal hematopoietic cells. This results in anemia, which is often normocytic and normochromic. The "pelger-huet" anomaly in neutrophils is a characteristic finding in this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Leukemia - While leukemia can cause myelophthisic anemia, it is not the most common cause.

**Option B:** Lymphoma - Lymphoma can also cause myelophthisic anemia, but it is not the most common cause.

**Option C:** Sickle cell disease - Sickle cell disease is a cause of hemolytic anemia, not myelophthisic anemia.

**Clinical Pearl / High-Yield Fact**
Remember that the "pelger-huet" anomaly in neutrophils is a characteristic finding in myelophthisic anemia, particularly in multiple myeloma.

**Correct Answer: D. Multiple myeloma**