**Question:** Increase in threshold level on applying a sub-threshold, slowly rising stimulus is known as:

**Core Concept:**
The concept being tested here is the concept of threshold in sensory physiology. Threshold is the minimum intensity stimulus required to elicit a response from a sensory system. Sub-threshold stimuli are those that are weaker than the threshold level, while a slowly rising stimulus refers to a stimulus that increases in intensity gradually.

**Why the Correct Answer is Right:**
The correct answer, **"Option A"**, refers to the phenomenon of increased threshold level when a sub-threshold, slowly rising stimulus is applied. This means that as the stimulus is made stronger (more slowly), the sensory system becomes more sensitive, and the threshold for response increases. This is an adaptive response, as it helps in preventing overstimulation and potential damage to the sensory receptor.

**Why Each Wrong Option is Incorrect:**

A. **"Option B"**: Increased threshold does not occur when a supra-threshold stimulus is applied. Supra-threshold stimuli are those that are above the threshold level, and applying a supra-threshold stimulus does not make the sensory system more sensitive, thereby not increasing the threshold.

C. **"Option C"**: Increased threshold does not occur when the stimulus is abruptly applied, rather than slowly. The adaptive response is specific to slowly increasing stimuli, as it allows the sensory system to adapt and prevent potential damage.

D. **"Option D"**: Increased threshold does not occur when a constant (non-changing) stimulus is applied, as opposed to a slowly increasing stimulus. The adaptive response is specific to stimuli that increase gradually, allowing the sensory system to adjust and protect itself.

**Clinical Pearl:**
Understanding this phenomenon helps in understanding how the body adapts to stimuli to protect itself from potential damage or overstimulation. In clinical scenarios, this knowledge can be applied to interpret patient complaints, such as a patient complaining of decreased sensitivity when a slowly increasing stimulus is applied, which could indicate a problem with the sensory system or the stimulus itself.