**Core Concept**
Gettler's test is a laboratory test used to determine the cause of death, specifically in cases of drowning. It involves measuring the concentration of sodium (Na+) in the left and right sides of the heart. This test relies on the principle that the sodium concentration in the blood is higher in the left side of the heart due to the mixing of blood from the entire body, whereas the right side of the heart receives deoxygenated blood with lower sodium levels.

**Why the Correct Answer is Right**
Gettler's test is based on the fact that in cases of drowning, the sodium levels in the blood can be altered due to the aspiration of water into the lungs. The test helps to differentiate between death by drowning and other causes by comparing the sodium levels in the left and right sides of the heart. The left side typically has a higher sodium concentration than the right side in cases of drowning. This is because the left side receives blood from the entire body, including the lungs, which have taken up sodium from the water aspirated during drowning.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Gettler's test is specifically used to detect drowning, not other causes of death such as poisoning or trauma.
**Option B:** This option is incorrect because Gettler's test is not used to detect death by electrocution, which involves different physiological mechanisms.
**Option C:** This option is incorrect because Gettler's test is not used to detect death by hanging, which involves asphyxiation rather than drowning.

**Clinical Pearl / High-Yield Fact**
Gettler's test is a classic example of a post-mortem test that can help determine the cause of death in cases where the circumstances are unclear. It highlights the importance of laboratory tests in forensic pathology.

**Correct Answer: D. Drowning**