**Core Concept**
Vital capacity (VC) is the maximum amount of air a person can exhale after a maximum inhalation. Timed vital capacity, or more specifically, Forced Expiratory Volume at 1 second (FEV1), measures the volume of air that can be exhaled in one second. The ratio of FEV1 to VC is used to assess the severity of obstructive lung diseases.

**Why the Correct Answer is Right**
A decrease in vital capacity with a normal timed vital capacity ratio (FEV1/VC) suggests a restrictive lung disease. In restrictive lung diseases, the lung's ability to expand is impaired, leading to a decrease in vital capacity. However, the FEV1, which measures the rate of air flow, remains normal because the airways are not obstructed. This is characteristic of conditions such as pulmonary fibrosis, where the lung tissue becomes scarred, leading to a reduction in lung volume.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because obstructive lung diseases, such as asthma or COPD, would typically present with a decreased FEV1/VC ratio, not a normal one.

**Option B:** This option is incorrect because a normal vital capacity with a decreased FEV1/VC ratio would suggest an obstructive lung disease, not a restrictive one.

**Option C:** This option is incorrect because a decrease in both vital capacity and FEV1/VC ratio would suggest a mixed restrictive and obstructive lung disease, not a pure restrictive disease.

**Clinical Pearl / High-Yield Fact**
In patients with restrictive lung disease, the lung's ability to expand is impaired, leading to a decrease in vital capacity. However, the FEV1, which measures the rate of air flow, remains normal because the airways are not obstructed.

**Correct Answer: C. Restrictive lung disease.**