**Core Concept**
Cyanosis is a condition characterized by a bluish discoloration of the skin and mucous membranes, typically due to decreased oxygenation of the blood. Methemoglobinemia is a condition where methemoglobin, a form of hemoglobin that cannot bind oxygen, accumulates in the blood.

**Why the Correct Answer is Right**
Cyanosis becomes clinically apparent when the concentration of methemoglobin exceeds 1.5 g/dL. This is because methemoglobin has a different spectral absorption curve than normal hemoglobin, allowing it to scatter light in a way that gives the skin a bluish hue. The increased levels of methemoglobin lead to a left shift in the oxygen-hemoglobin dissociation curve, impairing oxygen delivery to tissues. The presence of methemoglobin can be detected using the methemoglobin reduction test, which involves adding methylene blue to the blood sample.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because 0.5 g/dL is a concentration of methemoglobin that is not typically associated with clinically apparent cyanosis.
**Option B:** This option is incorrect because 0.1 g/dL is a very low concentration of methemoglobin and would not be expected to cause cyanosis.
**Option D:** This option is incorrect because 5 g/dL is an extremely high concentration of methemoglobin, which would likely cause severe tissue hypoxia and other systemic symptoms before cyanosis becomes clinically apparent.

**Clinical Pearl / High-Yield Fact**
Methemoglobinemia can be caused by a variety of factors, including exposure to certain medications (such as local anesthetics and antimalarial drugs), toxins (such as nitrates and nitrites), and genetic disorders (such as methemoglobinemia due to diaphorase deficiency).

**Correct Answer:** C. 1.5 g/dL