**Core Concept**
Arsenic poisoning, a type of heavy metal poisoning, can be diagnosed through various laboratory tests that detect arsenic levels or its metabolites in biological samples. Arsenic exposure can lead to chronic health issues, including skin lesions, cardiovascular disease, and diabetes.

**Why the Correct Answer is Right**
In chronic arsenic poisoning, samples like hair, nails, and urine can be sent for laboratory examination to detect arsenic levels. Hair and nail samples can store arsenic for extended periods, making them useful for assessing long-term exposure. Urine samples can provide a snapshot of current arsenic levels. These samples can be analyzed using techniques like atomic absorption spectroscopy (AAS) or inductively coupled plasma mass spectrometry (ICP-MS).

**Why Each Wrong Option is Incorrect**
**Option A:** Arsenic levels in blood are not typically used for diagnosing chronic arsenic poisoning, as arsenic is rapidly metabolized and cleared from the bloodstream. Blood samples may not accurately reflect long-term exposure.
**Option B:** Arsenic levels in stool can be sent for examination, although this is less common due to the difficulty in obtaining and processing stool samples.
**Option C:** Arsenic levels in water can be sent for examination, but this is not a biological sample from an individual, making it irrelevant to diagnosing chronic arsenic poisoning in a patient.

**Clinical Pearl / High-Yield Fact**
When considering laboratory tests for arsenic poisoning, it's essential to remember that urine samples are more relevant for acute exposure, while hair and nail samples are better suited for assessing long-term exposure.

**Correct Answer: A. Arsenic levels in blood are not typically used for diagnosing chronic arsenic poisoning, as arsenic is rapidly metabolized and cleared from the bloodstream.**