**Core Concept**
Sickle cell anemia is a genetic disorder caused by a mutation in the **HBB** gene, leading to the production of abnormal **hemoglobin S**. This results in **sickle-shaped red blood cells** that can obstruct small blood vessels. The underlying principle being tested is the **pathophysiology of sickle cell disease**.

**Why the Correct Answer is Right**
The correct answer is related to the characteristic features of sickle cell anemia, which include **chronic hemolysis**, **recurrent pain crises**, and **increased risk of infections**. The abnormal hemoglobin S causes red blood cells to become rigid and sickle-shaped under **low oxygen conditions**, leading to **vaso-occlusion** and tissue damage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary characteristic of sickle cell anemia.
**Option B:** This option is incorrect because it is not a specific feature of sickle cell anemia.
**Option C:** This option is incorrect because it is not directly related to the **pathophysiology of sickle cell disease**.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that sickle cell anemia is characterized by **recurrent episodes of pain** due to vaso-occlusion, and patients are at increased risk of **infections**, particularly with **Streptococcus pneumoniae**.

**Correct Answer:** D. Chronic hemolysis with episodic pain crises