## **Core Concept**
A right-to-left shunt occurs when deoxygenated blood bypasses the lungs and directly enters the systemic circulation, leading to decreased oxygen saturation in the arterial blood. The quantification of shunt fraction helps in assessing the severity of the shunt.

## **Why the Correct Answer is Right**
To calculate the percentage shunting of cardiac output, we can use the formula for shunt fraction (Qs/Qt), where Qs is the shunted blood and Qt is the total cardiac output. The formula using oxygen content is:
[ text{Shunt Fraction (Qs/Qt)} = frac{Cc - Ca}{Cc - Cv} ]
where:
- (Cc) is the oxygen content in pulmonary capillary blood (20 ml/100 ml),
- (Ca) is the oxygen content in arterial blood (18 ml/100 ml),
- (Cv) is the oxygen content in venous blood (14 ml/100 ml).

Substituting the given values:
[ text{Shunt Fraction} = frac{20 - 18}{20 - 14} = frac{2}{6} = frac{1}{3} = 0.33 text{ or } 33% ]

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the calculated shunt fraction does not match this value.
- **Option B:** This option is incorrect because, although close, it does not accurately represent the calculated shunt fraction of 33%.
- **Option D:** This option is incorrect as it suggests a much higher shunt fraction than calculated.

## **Clinical Pearl / High-Yield Fact**
A clinically significant shunt is often considered when the shunt fraction exceeds 20%. The presence of a right-to-left shunt indicates severe impairment of gas exchange, often requiring specific interventions such as oxygen therapy, and in severe cases, surgical correction.

## **Correct Answer:** . 33%