Lead time is defined is –

**Core Concept**
Lead time is a crucial concept in the field of medical screening and diagnosis. It refers to the length of time between the first detection of a disease through screening and the point at which the disease would have become clinically apparent if no screening had been performed. This concept is essential in understanding the benefits and limitations of disease screening.

**Why the Correct Answer is Right**
Lead time is a measure of the delay between the detection of a disease through screening and its clinical presentation. This concept is closely related to the idea of "surveillance bias," which can lead to an overestimation of the effectiveness of screening programs. The lead time is determined by the sensitivity and specificity of the screening test, as well as the natural history of the disease. For example, in the case of breast cancer, lead time is influenced by factors such as tumor size, growth rate, and the sensitivity of mammography.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it does not accurately define lead time. While it is related to the concept of disease detection, it does not specifically refer to the delay between screening and clinical presentation.

**Option B:**
This option is incorrect because it is a measure of disease progression, not lead time. It refers to the rate at which a disease progresses from its initial stages to its clinical presentation.

**Option C:**
This option is incorrect because it is a measure of disease incidence, not lead time. It refers to the frequency of new cases of a disease within a given population.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that lead time can lead to an overestimation of the effectiveness of screening programs if not properly accounted for. This is known as "surveillance bias," which can result in an overestimation of the benefits of screening.

**Correct Answer: A. Lead time is the length of time between the first detection of a disease through screening and the point at which the disease would have become clinically apparent if no screening had been performed.**

✓ Correct Answer: B. Time between point where the diagnosis is made by screening test to usual diagnosis.