**Core Concept**
Vitamin C excess can lead to an overload of ascorbic acid in the body, causing an imbalance in the metabolism of certain compounds. This can result in the accumulation of oxalic acid, a toxic compound that can cause kidney stones and other health issues.

**Why the Correct Answer is Right**
The correct answer is related to the metabolism of oxalic acid, which is a byproduct of ascorbic acid breakdown. In high doses, vitamin C can increase the production of oxalic acid, leading to its accumulation in the kidneys and subsequent formation of kidney stones. This is because ascorbic acid is metabolized into oxalic acid through the enzyme L-gulonolactone oxidase. In children, the kidneys may not be able to effectively excrete excess oxalic acid, increasing the risk of kidney stone formation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not directly related to the metabolism of ascorbic acid or the formation of kidney stones. While it is true that high doses of vitamin C can cause gastrointestinal upset, this is not the most significant concern in children with vitamin C excess.
**Option B:** This option is incorrect because it is not a direct consequence of vitamin C excess. While kidney damage can be caused by various factors, it is not a specific concern related to the metabolism of ascorbic acid.
**Option C:** This option is incorrect because it is not a known consequence of vitamin C excess. While vitamin C is important for collagen synthesis, excess levels do not directly affect the skin in this way.

**Clinical Pearl / High-Yield Fact**
In children, excessive intake of vitamin C can lead to an increased risk of kidney stone formation due to the accumulation of oxalic acid. This highlights the importance of monitoring vitamin C intake in pediatric patients to prevent this potentially debilitating condition.

**Correct Answer:** C.