**Core Concept**
Pasteurization is a process that kills harmful bacteria and extends the shelf life of milk by applying heat for a short period. This process is crucial in preventing milk-borne illnesses and ensuring the safety of dairy products.

**Why the Correct Answer is Right**
The correct answer is related to the detection of a specific enzyme that is inactivated by heat during pasteurization. Pasteurized milk is most commonly tested for alkaline phosphatase (ALP) activity. ALP is an enzyme produced by the mammary gland of cows, and its presence in milk indicates that the milk has not been pasteurized. When milk is pasteurized, the heat inactivates ALP, making it a reliable indicator of pasteurization.

**Why Each Wrong Option is Incorrect**
* **Option A:** Lactose content is not a reliable indicator of pasteurization, as lactose is a naturally occurring sugar in milk that is not affected by heat.
* **Option B:** Casein is a protein found in milk, but its presence or absence does not indicate pasteurization.
* **Option C:** Bacterial cultures can be used to test for contamination, but they do not specifically indicate pasteurization.

**Clinical Pearl / High-Yield Fact**
Remember that pasteurization is a critical step in ensuring the safety of dairy products. The use of alkaline phosphatase as an indicator of pasteurization is a widely accepted practice in the dairy industry.

**Correct Answer:** D.