**Core Concept**
Macular edema in non-proliferative diabetic retinopathy (NPDR) is characterized by the accumulation of fluid and lipids in the macula, leading to vision impairment. This condition is a result of damage to the blood-retinal barrier, allowing fluid to leak from the retinal capillaries into the macular tissue.

**Why the Correct Answer is Right**
Macular edema is the most common cause of vision loss in NPDR due to the central location of the macula and its high concentration of photoreceptors. In NPDR, the small blood vessels in the retina become leaky, causing fluid and lipids to accumulate in the macula. This accumulation of fluid can lead to macular thickening, distortion, and vision impairment. The macula is responsible for central vision, making macular edema a significant cause of vision loss in patients with NPDR.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitreous hemorrhage is more commonly associated with proliferative diabetic retinopathy (PDR), where new, fragile blood vessels can rupture and bleed into the vitreous gel. While vitreous hemorrhage can cause sudden vision loss, it is not the most common cause of vision loss in NPDR.

**Option C:** Detachment of the retina can occur in both NPDR and PDR, but it is not the most common cause of vision loss in NPDR. Retinal detachment is often associated with trauma, tumors, or other conditions that affect the vitreous or retina.

**Option D:** Subretinal hemorrhage can occur in various retinal conditions, including diabetic retinopathy, but it is not the most common cause of vision loss in NPDR. Subretinal hemorrhage can cause sudden vision loss due to bleeding into the subretinal space.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that macular edema is a treatable condition, and prompt intervention with anti-vascular endothelial growth factor (VEGF) therapy or laser photocoagulation can help improve vision in patients with NPDR.

**✓ Correct Answer: B. Macular edema**