## **Core Concept**
Reid's index is a measure used in the assessment of chronic bronchitis, which is a form of chronic obstructive pulmonary disease (COPD). It is defined as the ratio of the thickness of the submucosal mucus glands to the thickness of the wall between the epithelium and the cartilage in the bronchi. This index helps in quantifying the hypertrophy of mucus glands, a hallmark of chronic bronchitis.

## **Why the Correct Answer is Right**
The correct answer, **C. ratio of thickness of submucosal mucus gland to the thickness of wall between epithelium and cartilage**, accurately describes Reid's index. This measurement is specifically used to assess the severity of chronic bronchitis by evaluating the degree of mucus gland hypertrophy. The index is calculated by dividing the thickness of the submucosal mucus gland layer by the total thickness of the bronchial wall between the epithelium and the cartilage. A ratio of 0.5 or greater is generally considered indicative of chronic bronchitis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because it does not specify the correct structures or their relationship.
- **Option B:** Incorrect as it refers to a different pathological or physiological measurement not related to Reid's index.
- **Option D:** Incorrect because it also does not accurately describe Reid's index.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Reid's index is specifically used to diagnose and quantify the severity of **chronic bronchitis**, a condition characterized by a productive cough for at least 3 months in two consecutive years. This index is a valuable tool in distinguishing chronic bronchitis from other forms of COPD, such as emphysema.

## **Correct Answer:** C. ratio of thickness of submucosal mucus gland to the thickness of wall between epithelium and cartilage.