**Core Concept**
Gettler's test is a biochemical test used in forensic medicine to determine the chloride ion concentration in the blood and cerebrospinal fluid (CSF) of individuals who have died from drowning. It is based on the principle that the chloride ion concentration in the blood is higher than in the CSF in cases of drowning due to inhalation of water.

**Why the Correct Answer is Right**
In Gettler's test, the chloride ion concentration in the blood and CSF is measured. A higher chloride ion concentration in the blood compared to the CSF indicates that the individual died from drowning due to inhalation of water. This is because the lungs are filled with water, and the chloride ions from the water are absorbed into the bloodstream, increasing the chloride ion concentration. The test is used to differentiate between death by drowning and other causes of death.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to Gettler's test, which specifically measures chloride ion concentration in the blood and CSF.
* **Option B:** This option is incorrect because Gettler's test does not measure the pH level of the blood or CSF.
* **Option C:** This option is incorrect because Gettler's test is not used to detect the presence of a specific toxin or poison.

**Clinical Pearl / High-Yield Fact**
Gettler's test is a specific test used in forensic medicine to determine the cause of death, and it is particularly useful in cases where the individual died from drowning due to inhalation of water.

**Correct Answer: C. Chloride ion concentration in the blood and CSF.**