**Core Concept**
A low mean corpuscular volume (MCV) indicates a condition characterized by smaller red blood cells, typically seen in microcytic anemia. The normal red cell distribution width (RDW) suggests a relatively uniform size of red blood cells, ruling out causes of anisocytosis.

**Why the Correct Answer is Right**
A low MCV with a normal RDW is often seen in iron deficiency anemia (IDA). In IDA, the body's reduced iron availability leads to impaired hemoglobin synthesis, resulting in the production of smaller red blood cells. The normal RDW indicates that the red blood cells are relatively uniform in size, which is characteristic of IDA. The iron-dependent enzyme, delta-aminolevulinic acid synthase (ALAS), plays a crucial role in the early stages of heme synthesis. Reduced iron availability limits the activity of this enzyme, leading to the production of smaller red blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** An elevated RDW would suggest a condition with significant anisocytosis, such as megaloblastic anemia.
**Option B:** Thalassemia major typically presents with a low MCV and an elevated RDW due to the presence of various sizes of red blood cells.
**Option C:** Sideroblastic anemia is characterized by ringed sideroblasts in the bone marrow and often presents with a normal or elevated MCV.

**Clinical Pearl / High-Yield Fact**
A low MCV with a normal RDW is a classic presentation of iron deficiency anemia, which can be remembered using the mnemonic "MCV Low, RDW Normal, Think Iron Deficiency".

**Correct Answer:** A. Iron deficiency anemia.