**Core Concept**
Prussian blue is a chemical compound used as a staining agent in histopathology. It is a specific indicator for the presence of iron in tissues, particularly useful in diagnosing conditions like hemochromatosis and hemosiderosis.

**Why the Correct Answer is Right**
Prussian blue reacts with iron ions to form a blue pigment, allowing for the visualization of iron deposits in tissue sections. This reaction is based on the oxidation of ferrous ions to ferric ions, which then combine with the dye to produce the characteristic blue color. The specificity of Prussian blue for iron makes it a valuable tool in the diagnosis of iron-related disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Prussian blue is not typically used to detect copper, which is detected using other staining agents like rhodanine.
**Option B:** This option is incorrect because while Prussian blue may incidentally stain other substances, its primary and specific use is for detecting iron, not calcium.
**Option C:** This option is incorrect because Prussian blue is not used to detect melanin, which is detected using other staining agents like Fontana-Masson stain.

**Clinical Pearl / High-Yield Fact**
Prussian blue staining is a crucial diagnostic tool in the evaluation of patients with suspected iron overload or iron deficiency, particularly in conditions like hemochromatosis and thalassemia major.