**Core Concept**
Cyanosis is a clinical sign characterized by a bluish discoloration of the skin and mucous membranes, indicating an increased amount of reduced hemoglobin in the blood. It is a significant clinical finding that requires a specific concentration of reduced hemoglobin to be visible. The concentration of reduced hemoglobin necessary for cyanosis to occur depends on various factors, including the individual's skin pigmentation.

**Why the Correct Answer is Right**
The minimum concentration of reduced hemoglobin required for cyanosis to be visible is approximately 5 g/dL. This is because reduced hemoglobin has a different optical density than oxyhemoglobin, allowing it to scatter light and give the skin a bluish appearance. In individuals with darker skin pigmentation, more reduced hemoglobin is required to produce a noticeable cyanotic effect. The critical concentration of 5 g/dL is a general guideline, as it may vary depending on individual factors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific concentration of reduced hemoglobin, making it an ambiguous choice.
**Option B:** This option is incorrect because it suggests a lower concentration of reduced hemoglobin than the actual minimum required for cyanosis to be visible.
**Option C:** This option is incorrect because it suggests a higher concentration of reduced hemoglobin than the actual minimum required for cyanosis to be visible.

**Clinical Pearl / High-Yield Fact**
Cyanosis is often used as a clinical indicator of significant hypoxemia, but it is essential to remember that it may not be visible in individuals with darker skin pigmentation until the concentration of reduced hemoglobin reaches 5 g/dL or higher. This is why other clinical indicators, such as oxygen saturation levels, should also be used to assess hypoxemia.

**Correct Answer:** C. 5 g/dL.