**Core Concept**
Cyanosis is a clinical sign characterized by a bluish discoloration of the skin and mucous membranes, resulting from an increased amount of reduced hemoglobin in the blood. It is typically observed in conditions where there is a significant decrease in the oxygen-carrying capacity of the blood.

**Why the Correct Answer is Right**
Cyanosis occurs when the reduced hemoglobin concentration exceeds 5g/dL. This can be due to various causes such as respiratory or cardiac conditions, anemia, or conditions affecting the peripheral circulation. However, cyanosis is not a characteristic feature of conditions that primarily affect the oxygenation of the blood at the tissue level, such as hyperventilation syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because methemoglobinemia is a condition that can cause cyanosis due to the presence of methemoglobin, which is a form of hemoglobin that cannot bind oxygen.

**Option B:** This option is incorrect because carbon monoxide poisoning can cause cyanosis due to the formation of carboxyhemoglobin, which is a stable complex that cannot bind oxygen.

**Option C:** This option is incorrect because polycythemia vera is a condition characterized by an increased red blood cell mass, which can lead to increased reduced hemoglobin and consequently, cyanosis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that not all cases of cyanosis are due to decreased oxygenation of the blood. In some cases, cyanosis can be a sign of other conditions, such as methemoglobinemia or carbon monoxide poisoning.

**Correct Answer:** D.