**Core Concept**
Autosplenectomy refers to the spontaneous infarction and atrophy of the spleen, often resulting from recurrent episodes of sickling in patients with sickle cell disease. This process is due to the obstruction of the splenic vasculature by sickled erythrocytes, leading to ischemia and ultimately, splenic infarction.

**Why the Correct Answer is Right**
In patients with sickle cell disease, repeated episodes of sickling can cause damage to the splenic vasculature, leading to obstruction of the splenic arteries and veins. This results in ischemia and infarction of the spleen, ultimately leading to autosplenectomy. The spleen's function, including immune response and erythrocyte filtration, is impaired due to this process.

**Why Each Wrong Option is Incorrect**
**Option A:** Autosplenectomy is not typically seen in patients with thalassemia major.
**Option B:** Autosplenectomy is not a characteristic feature of hereditary spherocytosis.
**Option C:** Autosplenectomy is not a direct result of portal hypertension.

**Clinical Pearl / High-Yield Fact**
In patients with sickle cell disease, autosplenectomy often occurs before the age of 5, resulting in a loss of splenic function and increased susceptibility to infections.

**Correct Answer: C. Sickle cell disease.**