**Question:** Which of the following is markedly decreased in restrictive lung disease -

A. FEV1
B. FVC
C. FEV1/FVC ratio
D. Total lung capacity (TLC)

**Core Concept:**
In restrictive lung diseases, the lung volume is diminished due to various factors such as decreased compliance, reduced elastic recoil, and reduced functional residual capacity (FRC). These conditions result in a decrease in total lung capacity (TLC), which is the maximum amount of air a healthy lung can hold.

**Why the Correct Answer is Right:**
Total lung capacity (TLC) is markedly decreased in restrictive lung disease because the disease affects the overall lung volume, making it challenging for the individual to breathe in and out completely. In this context, restrictive lung disease refers to conditions that limit the lung's ability to expand, leading to decreased TLC.

**Why Each Wrong Option is Incorrect:**
A. FEV1 (Forced Expiratory Volume in the first second) is an important measure of lung function, but it is not the primary indicator of restrictive lung disease. In restrictive lung disease, FEV1 may be preserved or even increased due to the shift of air from the alveoli to the larger airways, which results in larger FEV1 values.

B. FVC (Forced Vital Capacity) is another measure of lung function, but it does not specifically address the restrictive lung disease condition. As with FEV1, FVC may be preserved or even increased in restrictive lung disease due to the same airway-filling phenomenon.

C. FEV1/FVC ratio is used for diagnosing obstructive lung disease, not restrictive lung disease. In restrictive lung disease, FEV1/FVC may be reduced or normal due to the above-mentioned airway-filling effect.

**Clinical Pearl:**
Restrictive lung diseases, such as pulmonary fibrosis, restrictive thoracic cage deformities, and neuromuscular disorders, are characterized by a decrease in TLC due to reduced lung expansion. Assessing TLC is essential in diagnosing restrictive lung disease, as the other measures, such as FEV1 and FVC, may not be reduced due to the airway-filling effect.

**Correct Answer:** D. Total lung capacity (TLC) is the correct measure to identify a reduction in lung expansion, which is characteristic of restrictive lung diseases. Assessing TLC is essential in diagnosing restrictive lung disease, as the other measures, such as FEV1 and FVC, may not be reduced due to the airway-filling effect.