**Core Concept**
Insulin-dependent diabetes mellitus (IDDM) can lead to complications such as diabetic retinopathy, which affects the blood vessels in the retina and can cause vision loss. This condition is a microvascular complication of diabetes.

**Why the Correct Answer is Right**
The patient's symptoms of gradually progressive painless loss of vision are indicative of diabetic retinopathy, specifically proliferative diabetic retinopathy (PDR). In PDR, the damaged blood vessels in the retina cause the formation of new, fragile blood vessels that can leak and cause scarring, leading to vision loss. The pathophysiology involves the increased levels of glucose in the blood, which damage the endothelial cells lining the blood vessels, leading to their weakening and eventual rupture.

**Why Each Wrong Option is Incorrect**
**Option A:** Diabetic neuropathy is a common complication of diabetes, but it typically presents with pain, numbness, or tingling in the extremities, rather than progressive painless loss of vision.
**Option B:** Hypertensive retinopathy is a condition that can cause vision loss due to high blood pressure, but it is not directly related to diabetes and typically presents with different symptoms, such as flashes of light and floaters.
**Option C:** Glaucoma is an eye condition that can cause vision loss, but it is not directly related to diabetes and typically presents with symptoms such as eye pain, redness, and vision changes.

**Clinical Pearl / High-Yield Fact**
The "nocturnal pulse oximetry test" can be used to diagnose diabetic retinopathy, as patients with PDR often have a significant drop in oxygen saturation during sleep.

**Correct Answer:** C.