**Core Concept**
Severe marginal breakdown in a tooth restored with a low copper amalgam filling is a clinical manifestation related to the material properties of the amalgam. Low copper amalgam fillings have reduced corrosion resistance and higher mercury release compared to high copper amalgam, which can lead to marginal deterioration.

**Why the Correct Answer is Right**
The primary reason for marginal breakdown in low copper amalgam fillings is the increased corrosion of the amalgam. This is due to the higher reactivity of the mercury and tin components, which can lead to the formation of hydrogen gas and the release of mercury ions. As a result, the amalgam undergoes corrosion, leading to marginal breakdown and the formation of microleakage.

**Why Each Wrong Option is Incorrect**
**Option A:** Galvanic corrosion is a possible cause of marginal breakdown, but it is not the primary reason for low copper amalgam fillings. This can occur when two dissimilar metals are in contact, but it is not the main factor in this scenario.

**Option B:** Porosity in the amalgam is a common issue, but it is not directly related to the low copper content. While porosity can contribute to marginal breakdown, it is not the primary cause in this case.

**Option C:** High copper amalgam fillings actually have a lower incidence of marginal breakdown due to their improved corrosion resistance. This option is incorrect because it is the opposite of what is being asked.

**Clinical Pearl / High-Yield Fact**
To minimize marginal breakdown, dentists should aim for a high copper amalgam filling with a copper content of at least 7-10%. This can help reduce corrosion and improve the overall durability of the restoration.

**Correct Answer:** C. High copper amalgam fillings have improved corrosion resistance and are less likely to cause marginal breakdown compared to low copper amalgam fillings.