**Core Concept**
Avascular nonunion of fracture refers to a type of fracture that fails to heal due to a lack of blood supply to the affected bone. This can occur due to various reasons such as insufficient blood flow, bone fragmentation, or the presence of a fracture gap. Avascular nonunion is a serious complication that can lead to persistent pain, deformity, and impaired mobility.

**Why the Correct Answer is Right**
The correct answer is an exception to the types of avascular nonunion of fracture. Avascular nonunion can be classified into different types based on the underlying cause, including:

* Atrophic nonunion: characterized by a lack of blood supply and a decrease in bone density
* Hypertrophic nonunion: characterized by a lack of blood supply but with an excessive callus formation
* Osteoporotic nonunion: characterized by a weakened bone structure and a lack of blood supply
* Osteonecrotic nonunion: characterized by a lack of blood supply and bone death

**Why Each Wrong Option is Incorrect**
**Option A:** Atrophic nonunion is a type of avascular nonunion, so it is not the correct answer. Atrophic nonunion is characterized by a lack of blood supply and a decrease in bone density.
**Option B:** Hypertrophic nonunion is a type of avascular nonunion, so it is not the correct answer. Hypertrophic nonunion is characterized by a lack of blood supply but with an excessive callus formation.
**Option C:** Osteoporotic nonunion is a type of avascular nonunion, so it is not the correct answer. Osteoporotic nonunion is characterized by a weakened bone structure and a lack of blood supply.
**Option D:** OHO (Osteochondral fracture) is not a type of avascular nonunion of fracture. OHO refers to a type of fracture that involves the cartilage and bone, often seen in osteochondral lesions.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between various types of avascular nonunion to provide appropriate treatment. Treatment options for avascular nonunion may include surgical intervention, such as bone grafting or internal fixation, or non-surgical options like immobilization and physical therapy.

**Correct Answer:** D. OHO (Osteochondral fracture)