**Core Concept:**
Medullary index (MI) is a measure of the proportion of medulla (central cellular region) to the total cross-sectional area of hair shaft in hair follicles. A higher MI indicates a larger medulla, which results in a softer and more flexible hair. The medulla contains keratinocytes and lipids which contribute to these properties.

**Why the Correct Answer is Right:**
The correct answer is **D**, as the medullary index is calculated using the formula:

MI = (Medulla Area / Total Area) x 100

A higher MI indicates a larger proportion of the medulla to the total cross-sectional area of the hair shaft, resulting in softer and more flexible hair.

**Why Each Wrong Option is Incorrect:**
A) The wrong answer **A** is incorrect because it does not account for the total cross-sectional area of the hair shaft, which is crucial in determining the hair's properties.
B) The wrong answer **B** is incorrect because it only considers the medulla area, disregarding the total area, which is essential for understanding the hair's characteristics.
C) The wrong answer **C** is incorrect as it does not follow the formula used to calculate the MI and does not consider the total area of the hair shaft, leading to an inaccurate representation of the hair's properties.

**Why the Correct Answer is Right:**
The correct answer **D** accurately represents the relationship between the medulla area and the total area of the hair shaft, reflecting the hair's softness and flexibility.

**Clinical Pearl:**
Understanding the medullary index is essential in dermatology as it helps in correlating hair characteristics to genetic disorders affecting hair growth, such as androgenetic alopecia (male pattern baldness) and trichotillomania (hair pulling disorder), where changes in hair shaft properties can be observed.