**Core Concept**
Diabetic foot ulcers are a common complication of diabetes mellitus, often resulting from neuropathy, peripheral artery disease, and poor foot care. These ulcers most frequently occur on the plantar surface of the foot, where pressure and shear forces are highest.

**Why the Correct Answer is Right**
The most common site for diabetic foot ulcers is the plantar surface of the foot, particularly under the metatarsal heads. This is due to the high pressure and shear forces exerted on the skin in this area, leading to callus formation and subsequent ulceration. The neuropathy associated with diabetes can also contribute to the development of ulcers, as patients may not feel pain or discomfort in the affected area.

**Why Each Wrong Option is Incorrect**
* **Option A:** Medial malleolus - This area is less likely to develop ulcers due to the lower pressure and shear forces compared to the plantar surface.
* **Option B:** Lateral malleolus - Similar to the medial malleolus, this area is less prone to ulcers due to lower pressure and shear forces.
* **Option C:** Heel - While the heel can develop ulcers, it is not the most common site, as the pressure and shear forces are not as high as under the metatarsal heads.
* **Option D:** Head of toes - Ulcers can occur on the toes, but they are less common than those on the plantar surface of the foot.

**Clinical Pearl / High-Yield Fact**
When assessing a patient with diabetes for foot ulcers, it is essential to inspect the plantar surface of the foot, particularly under the metatarsal heads, and to educate patients on proper foot care, including regular inspections and avoidance of high-pressure areas.

**Correct Answer:** D. Head of metatarsal