**Core Concept**
Methotrexate is a chemotherapeutic agent and a disease-modifying antirheumatic drug (DMARD) that inhibits dihydrofolate reductase (DHFR), an enzyme crucial for DNA synthesis and repair. By blocking this enzyme, methotrexate reduces the proliferation of rapidly dividing cells, such as cancer cells and inflammatory cells.

**Why the Correct Answer is Right**
Methotrexate is used in various conditions, including psoriasis, rheumatoid arthritis, and ankylosing spondylitis, where it helps to reduce inflammation and slow disease progression. However, it is not typically used in the treatment of sickle cell anemia, as this condition involves a different pathophysiological mechanism. Sickle cell anemia is characterized by a hemoglobinopathy, where the abnormal hemoglobin (HbS) causes red blood cells to become misshapen and prone to premature destruction. Methotrexate's mechanism of action does not directly address the underlying cause of sickle cell anemia.

**Why Each Wrong Option is Incorrect**
**Option B:** Methotrexate is indeed used in the treatment of psoriasis, particularly for its ability to reduce inflammation and slow disease progression.
**Option C:** Methotrexate is a commonly used DMARD in rheumatoid arthritis, where it helps to reduce inflammation and slow joint damage.
**Option D:** Methotrexate can also be used in the treatment of ankylosing spondylitis, where it helps to reduce inflammation and slow disease progression.

**Clinical Pearl / High-Yield Fact**
When using methotrexate, it's essential to monitor patients for potential adverse effects, such as liver toxicity and bone marrow suppression, and to adjust doses accordingly. Additionally, methotrexate can interact with other medications, such as folic acid supplements, which can affect its efficacy.

**✓ Correct Answer: A. > Sickle cell anemia**