## Core Concept
Sickle cell disease is a genetic disorder that affects hemoglobin production, leading to abnormal hemoglobin called hemoglobin S (HbS). This abnormal hemoglobin causes red blood cells (RBCs) to change shape under certain conditions. The underlying principle being tested here is the pathophysiology of sickle cell disease, specifically the cause of RBC sickling.

## Why the Correct Answer is Right
The correct answer, , is related to the polymerization of deoxyhemoglobin S. When HbS is deoxygenated, it polymerizes and forms long, fibrous aggregates that distort the RBC into a sickle shape. This process is triggered by low oxygen levels, dehydration, or increased concentration of 2,3-bisphosphoglycerate (2,3-BPG), which facilitates the polymerization of deoxyhemoglobin S. The polymerization of deoxyhemoglobin S is the primary cause of RBC sickling in sickle cell disease.

## Why Each Wrong Option is Incorrect
**Option A:** - This option is incorrect because while changes in pH can affect hemoglobin's affinity for oxygen, the primary cause of sickling is not directly related to altered pH levels.

**Option B:** - This option is incorrect because, although increased 2,3-BPG can facilitate the polymerization of deoxyhemoglobin S by reducing hemoglobin's affinity for oxygen, it is not the direct cause of sickling.

**Option C:** - This option might seem plausible due to the role of calcium in cell signaling and membrane changes, but it is not directly implicated as the primary cause of RBC sickling in sickle cell disease.

## Clinical Pearl / High-Yield Fact
A key point to remember is that sickling of RBCs in sickle cell disease is primarily triggered by conditions that lead to deoxygenation of hemoglobin S, such as high altitude, dehydration, or infection. Understanding this helps in managing patients with sickle cell disease by avoiding triggers that can precipitate sickling crises.

**Correct Answer: D. polymerization of deoxyhemoglobin S**