**Core Concept**
The Reinsch test is a chemical test used to detect the presence of certain heavy metals, particularly arsenic, in biological samples. This test relies on the reaction between arsenic and bismuth subsalicylate to form a white precipitate.

**Why the Correct Answer is Right**
The Reinsch test involves mixing the biological sample with bismuth subsalicylate and then adding a strong acid. In the presence of arsenic, a white precipitate of bismuth arsenate forms, indicating a positive result. This reaction is specific to arsenic due to its unique chemical properties, which allow it to form a complex with the bismuth subsalicylate. The test is commonly used in forensic medicine to detect arsenic poisoning.

**Why Each Wrong Option is Incorrect**
**Option B:** Lead is not detected by the Reinsch test, as it does not form a similar complex with bismuth subsalicylate. Lead poisoning is typically diagnosed using other methods, such as atomic absorption spectroscopy or mass spectrometry.
**Option C:** Iron is also not detected by the Reinsch test, as it does not react with bismuth subsalicylate to form a precipitate. Iron poisoning is usually diagnosed using other methods, such as serum iron levels or stool tests.
**Option D:** Copper sulphate is not detected by the Reinsch test, as it does not form a complex with bismuth subsalicylate. Copper poisoning is typically diagnosed using other methods, such as serum copper levels or urine tests.

**Clinical Pearl / High-Yield Fact**
The Reinsch test is a specific and sensitive method for detecting arsenic poisoning, but it requires careful handling and interpretation of results. It is essential to note that the test can be affected by the presence of other heavy metals or substances that may interfere with the reaction.

**✓ Correct Answer: A. Arsenic**