**Core Concept**
Cyanosis is a clinical sign characterized by a bluish discoloration of the skin and mucous membranes, indicating inadequate oxygenation of the blood. The improvement of cyanosis with 100% oxygen is a key diagnostic clue, often used to differentiate between various causes.

**Why the Correct Answer is Right**
In cases of cyanosis caused by a right-to-left shunt, such as tetralogy of Fallot, 100% oxygen will not improve the cyanosis because the deoxygenated blood bypasses the lungs and directly enters the systemic circulation. This is due to the anatomical abnormality, where the aorta arises from the right ventricle, allowing deoxygenated blood to mix with oxygenated blood.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cyanosis caused by anemia or carbon monoxide poisoning will improve with 100% oxygen.
* **Option B:** Cyanosis in patients with respiratory failure or chronic obstructive pulmonary disease (COPD) will improve with 100% oxygen due to increased oxygenation of the blood.
* **Option C:** Cyanosis in patients with cardiac failure or myocardial infarction may not improve with 100% oxygen, but this is not the primary cause of cyanosis in this scenario.

**Clinical Pearl / High-Yield Fact**
Remember that the improvement of cyanosis with 100% oxygen is a key diagnostic clue, but it is essential to consider the underlying cause and the patient's overall clinical presentation.

**Correct Answer: C. Cyanosis in cases of right-to-left shunt will not improve with 100% oxygen.**