**Core Concept**
Cyanosis is a clinical sign characterized by a bluish discoloration of the skin and mucous membranes due to the presence of reduced hemoglobin (deoxyhemoglobin) in the blood. The minimum concentration of reduced hemoglobin required for cyanosis to be visible is a critical concept in medicine, as it helps clinicians diagnose and manage various conditions affecting the respiratory and cardiovascular systems.

**Why the Correct Answer is Right**
The minimum concentration of reduced hemoglobin required for cyanosis is approximately 5g/dL. This is because when the concentration of deoxyhemoglobin in the blood reaches a certain threshold, it starts to absorb light and scatter it, giving the skin and mucous membranes a bluish appearance. This phenomenon is known as the "critical concentration" of deoxyhemoglobin. The critical concentration is influenced by factors such as the oxygenation status of the blood, the concentration of other pigments like carboxyhemoglobin, and the thickness of the skin and mucous membranes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the minimum concentration of reduced hemoglobin required for cyanosis is not 1g/dL. At this concentration, deoxyhemoglobin would not be visible on the skin or mucous membranes.

**Option B:** This option is incorrect because the minimum concentration of reduced hemoglobin required for cyanosis is not 10g/dL. At this concentration, deoxyhemoglobin would be visible on the skin and mucous membranes, but it is not the minimum concentration required for cyanosis to be clinically apparent.

**Option D:** This option is incorrect because the minimum concentration of reduced hemoglobin required for cyanosis is not 20g/dL. At this concentration, deoxyhemoglobin would be visible on the skin and mucous membranes, but it is not the minimum concentration required for cyanosis to be clinically apparent.

**Clinical Pearl / High-Yield Fact**
Cyanosis is a clinical sign that can be influenced by various factors, including the concentration of deoxyhemoglobin, the oxygenation status of the blood, and the concentration of other pigments like carboxyhemoglobin. Clinicians should be aware of these factors when diagnosing and managing patients with cyanosis.

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