**Question:** Increased Reid's index is increased in which of the following:

A. Pulmonary arterial hypertension
B. Coronary artery disease
C. Systemic sclerosis
D. Pulmonary embolism

**Core Concept:**

Reid's index, also known as the pulmonary artery to aorta ratio, is a measure used to assess the diameter of the pulmonary artery relative to the aorta. This ratio is crucial in evaluating various cardiovascular conditions, as it reflects the hemodynamic changes within the pulmonary vasculature.

**Why the Correct Answer is Right:**

D. Pulmonary embolism: In cases of pulmonary embolism, there is an increase in pulmonary vascular resistance, leading to vasoconstriction and subsequent dilation of the pulmonary artery. Consequently, the ratio of pulmonary artery to aorta diameter increases, making option D the correct answer.

**Why Each Wrong Option is Incorrect:**

A. Pulmonary arterial hypertension: While pulmonary arterial hypertension does lead to increased pulmonary vascular resistance and pulmonary artery dilation, it also results in the development of right ventricular hypertrophy and dilation. This would decrease the pulmonary artery to aorta ratio, making option A incorrect.

B. Coronary artery disease: Coronary artery disease primarily affects the coronary arteries and does not significantly impact the pulmonary vasculature or Reid's index. Therefore, option B is incorrect.

C. Systemic sclerosis: Systemic sclerosis is a connective tissue disease that primarily affects the skin and internal organs, not the cardiovascular system. Consequently, it does not directly influence Reid's index, making option C incorrect.

**Clinical Pearl:**

A key clinical pearl that should be remembered is that the ratio of pulmonary artery to aorta diameter can be useful in differentiating between various cardiac and pulmonary diseases. An elevated Reid's index may indicate pulmonary embolism, while a decreased index might point towards pulmonary arterial hypertension. Understanding these ratios can aid in guiding the appropriate diagnostic and therapeutic decisions for patients presenting with cardiovascular symptoms and signs.