A 52-year-old male from rural Bihar presents with progressive swelling of the left lower limb for 8 years. On examination, the skin is thickened, non-pitting, and the limb circumference is significantly enlarged. When you attempt to pinch the skin fold at the base of the 2nd toe (marked **D** in the diagram), you are unable to do so. Which of the following is the most likely diagnosis and the clinical significance of the finding marked **D**?