**Core Concept**
Breslau's second life test is a forensic technique used to determine the time elapsed since death, specifically focusing on the decomposition process of the gastrointestinal tract. This test is based on the observation that the stomach and intestines undergo distinct changes after death, which can be used to estimate the post-mortem interval.

**Why the Correct Answer is Right**
The stomach and intestines are the primary organs involved in Breslau's second life test. After death, the stomach undergoes autolysis, leading to the breakdown of its mucosal lining and the release of digestive enzymes. These enzymes cause the stomach contents to mix with the intestinal contents, resulting in a characteristic mixture that can be analyzed to estimate the time elapsed since death. The intestines also undergo changes, including putrefaction, which can be used to support the estimate. The brain, heart, and lungs are not directly involved in this test.

**Why Each Wrong Option is Incorrect**
**Option A:** The brain is not tested in Breslau's second life test, as it is not directly involved in the decomposition process of the gastrointestinal tract.
**Option B:** The heart is not tested in this forensic technique, as it does not undergo changes that can be used to estimate the post-mortem interval.
**Option C:** The lung is not tested in Breslau's second life test, as it is not directly involved in the decomposition process of the gastrointestinal tract.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Breslau's second life test is a secondary method to estimate the post-mortem interval, and it should be used in conjunction with other forensic techniques, such as the examination of the stomach and intestinal contents, to obtain a more accurate estimate.

**✓ Correct Answer: D. Stomach and Intestine**