## **Core Concept**
Salivary amylase, also known as ptyalin, is an enzyme produced in the salivary glands that plays a crucial role in the digestion of carbohydrates. It breaks down starches into simpler sugars. The activity of salivary amylase is influenced by factors such as pH and temperature.

## **Why the Correct Answer is Right**
The correct answer, **D. Low pH**, indicates that salivary amylase is inactivated by low pH. This is because salivary amylase is most active at a slightly alkaline to neutral pH (around pH 7). When exposed to acidic conditions, such as those found in the stomach with the presence of gastric acid (HCl), the enzyme's activity is significantly reduced or completely inactivated. This is relevant because the acidic environment of the stomach is designed to activate digestive enzymes like pepsinogen to pepsin, which then breaks down proteins, but it is not suitable for the activity of salivary amylase.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while extremely high temperatures can denature and inactivate salivary amylase, the question seems to be looking for a more physiological or direct regulatory mechanism related to its inactivation in the context of digestion.
- **Option B:** This option is incorrect because boiling (high temperature) would indeed denature and inactivate salivary amylase, but similar to option A, it doesn't directly relate to physiological conditions encountered during digestion.
- **Option C:** This option is incorrect because it does not provide a clear mechanism or condition related to the inactivation of salivary amylase.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the digestive process is highly compartmentalized and regulated by the body's internal environment. For instance, the change in pH from the mouth (where salivary amylase is active) to the stomach (where it is inactivated) is crucial for the sequential activation and deactivation of digestive enzymes, ensuring efficient digestion and absorption of nutrients.

## **Correct Answer:** D. Low pH.