**Core Concept**
The oxygen dissociation curve is a graphical representation of the relationship between the partial pressure of oxygen (pO2) in the blood and the percentage of hemoglobin (Hb) that is saturated with oxygen. It is a critical concept in respiratory physiology, reflecting the balance between oxygen delivery to tissues and the need for oxygen by the body.

**Why the Correct Answer is Right**
The oxygen dissociation curve is sigmoidal in shape due to the cooperative binding of oxygen to hemoglobin. This means that as the partial pressure of oxygen increases, the affinity of hemoglobin for oxygen decreases, allowing more oxygen to be released to tissues. The Bohr effect describes how a decrease in pH (acidosis) or an increase in CO2 levels also decreases the affinity of hemoglobin for oxygen, facilitating oxygen release to tissues. The curve is also influenced by temperature, with an increase in temperature shifting the curve to the right, indicating a decreased affinity for oxygen.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the sigmoidal shape of the oxygen dissociation curve is a result of the cooperative binding of oxygen to hemoglobin, not a result of the binding of oxygen to myoglobin.
**Option B:** This option is incorrect because the Bohr effect, which describes how pH and CO2 levels affect the affinity of hemoglobin for oxygen, is not related to the shape of the oxygen dissociation curve.
**Option C:** This option is incorrect because the temperature dependence of the oxygen dissociation curve is a result of changes in the affinity of hemoglobin for oxygen, not a result of changes in the shape of the curve.
**Option D:** This option is incorrect because the oxygen dissociation curve is not a straight line, but rather a sigmoidal curve that reflects the cooperative binding of oxygen to hemoglobin.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the oxygen dissociation curve is influenced by multiple factors, including pH, CO2 levels, and temperature. This means that a patient with acidosis or hypothermia may have a shifted oxygen dissociation curve, indicating a decreased affinity for oxygen and an increased need for oxygen delivery to tissues.

**Correct Answer: D. The oxygen dissociation curve is a straight line.**