**Core Concept**
The clinical presentation suggests a condition affecting the liver and the nervous system, with symptoms such as difficulty walking, behavioral disturbances, and bluish pigmentation over the lunula, which could be indicative of **Wilson's disease**, a genetic disorder characterized by excessive accumulation of **copper** in the body.

**Why the Correct Answer is Right**
Wilson's disease leads to copper accumulation in the liver, brain, and other organs, causing damage. The bluish pigmentation over the lunula (a part of the nail) is known as **Kayser-Fleischer rings** when it occurs in the cornea, a common finding in Wilson's disease. The recent history of prolonged jaundice also points towards liver involvement, which is consistent with Wilson's disease. The next logical step in diagnosis would involve investigating for copper metabolism abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is not provided, so we cannot assess its correctness.
**Option B:** Similarly, without the option details, we cannot evaluate its appropriateness.
**Option C:** And this option is also not given for evaluation.
**Option D:** Again, lacking the specific details of this option, it cannot be directly addressed.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Kayser-Fleischer rings** and **neurological symptoms** in the context of liver disease should prompt an investigation for Wilson's disease. Early diagnosis is crucial as it allows for the initiation of treatment that can prevent further progression of the disease.

**Correct Answer:** D. 24-hour urinary copper estimation