**Core Concept**
The patient's symptoms of progressive painless loss of vision are suggestive of diabetic retinopathy, a common complication of Insulin Dependent Diabetes Mellitus (IDDM). Diabetic retinopathy occurs due to chronic hyperglycemia leading to microvascular damage in the retina.

**Why the Correct Answer is Right**
Prolonged exposure to high glucose levels in patients with IDDM can cause non-enzymatic glycosylation of collagen and elastin in the retinal blood vessels, leading to their thickening and eventual closure. This results in ischemia and subsequent neovascularization, which can cause retinal detachment and vision loss. The patient's 10-year history of IDDM increases the likelihood of developing advanced diabetic retinopathy.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not directly related to the patient's symptoms. While hypertension can contribute to retinopathy, it is not the most likely cause in this case.
* **Option B:** This option is incorrect because the patient's symptoms do not suggest acute angle-closure glaucoma, which typically presents with severe eye pain, headache, and nausea.
* **Option C:** This option is incorrect because the patient's symptoms do not suggest a posterior vitreous detachment, which typically presents with sudden flashes of light and floaters.

**Clinical Pearl / High-Yield Fact**
In patients with IDDM, the risk of developing diabetic retinopathy increases with the duration of diabetes. Regular monitoring of blood glucose levels and annual eye exams can help detect retinopathy early and prevent vision loss.

**Correct Answer:** C.