**Core Concept**
Maturity onset diabetes mellitus refers to type 2 diabetes, which can lead to diabetic retinopathy, a condition affecting the blood vessels in the retina. **Diabetic retinopathy** can cause blindness if left untreated. Regular **ophthalmoscopy** is essential for early detection.

**Why the Correct Answer is Right**
The correct timing for ophthalmoscopy in patients with maturity onset diabetes mellitus is at the time of diagnosis, as **diabetic retinopathy** can be present even before the diagnosis of diabetes. This allows for early detection and treatment, reducing the risk of vision loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it may delay detection.
**Option B:** Incorrect as it is not the standard recommendation.
**Option D:** Incorrect because it is too late for early detection.

**Clinical Pearl / High-Yield Fact**
Remember, **diabetic retinopathy** is a leading cause of blindness in adults, and early detection through **ophthalmoscopy** at diagnosis can significantly improve outcomes.

**Correct Answer:** D. at the time of diagnosis.