**Core Concept**
The "Hair on End" appearance is a radiological sign characterized by the vertical trabeculation of the bone, resembling hairs standing upright. This phenomenon is typically observed in conditions associated with bone marrow expansion or hyperplasia.

**Why the Correct Answer is Right**
Thalassemia is a genetic disorder that leads to ineffective erythropoiesis and subsequent bone marrow expansion. As a result, the bone marrow undergoes hyperplasia, causing an increase in the number of trabeculae, which appear as vertical lines on radiographs. This "Hair on End" appearance is a classic radiological feature of thalassemia, particularly in the pelvis and long bones. The bone marrow expansion in thalassemia is a compensatory mechanism to increase erythropoiesis and meet the body's demands for red blood cells.

**Why Each Wrong Option is Incorrect**
**Option B:** Scurvy is caused by vitamin C deficiency, leading to impaired collagen synthesis and osteoid formation, resulting in a different radiological appearance known as "Trummerfeld zone" or "cystic changes" in bones.
**Option C:** Rickets is a disease of bone development caused by vitamin D deficiency, leading to softening of the bones and characteristic radiological features such as "rachitic rosary" or "cupping" of the metaphyses.
**Option D:** Hemochromatosis is a genetic disorder leading to iron overload, which can cause changes in the bone marrow and liver, but the "Hair on End" appearance is not a characteristic feature of this condition.

**Clinical Pearl / High-Yield Fact**
In radiology, the "Hair on End" appearance is not limited to thalassemia; it can also be seen in other conditions associated with bone marrow expansion, such as leukemia, lymphoma, and myeloproliferative disorders.

✓ Correct Answer: A. Thalassemia