## Core Concept
Nonunion in orthopedics refers to the failure of a fractured bone to heal properly, resulting in persistent separation of the bone fragments. This condition can significantly impact the patient's quality of life, causing chronic pain, limited mobility, and psychological distress. The most common causes of nonunion are related to factors that interfere with the normal bone healing process.

## Why the Correct Answer is Right
The correct answer, **Infection**, is a well-recognized cause of nonunion. When a fracture site becomes infected, the presence of bacteria and the ensuing inflammatory response can damage the delicate network of blood vessels and osteogenic cells necessary for bone repair. This can lead to a failure in the healing process, resulting in nonunion. Infection disrupts the normal phases of bone healing (inflammation, soft callus formation, hard callus formation, and bony remodeling), making it difficult for the bone to heal properly.

## Why Each Wrong Option is Incorrect
- **Option A: Poor blood supply** - While poor blood supply is indeed a critical factor that can lead to nonunion, as it limits the delivery of oxygen, nutrients, and cells necessary for bone healing, it is not listed as the correct answer here. However, it's worth noting that poor blood supply is a very common cause, often cited as the most common reason.
- **Option B: Inadequate immobilization** - Inadequate immobilization can indeed prevent proper healing of a fracture by allowing excessive movement at the fracture site, which can disrupt the formation of a stable callus. However, with modern surgical techniques and orthopedic devices, this is less commonly the primary cause of nonunion compared to biological factors like infection.
- **Option C: Malnutrition** - Malnutrition can impair the healing process by affecting the availability of essential nutrients and vitamins necessary for bone repair. However, it is not as directly impactful on the fracture site as infection.

## Clinical Pearl / High-Yield Fact
A key point to remember is that infection is a preventable cause of nonunion. Proper wound care, use of prophylactic antibiotics in certain cases, and early treatment of any developing infection can significantly reduce the risk of nonunion. Clinicians should always be vigilant for signs of infection, such as redness, swelling, warmth, and purulent drainage, especially in the post-operative period.

## Correct Answer: D. Infection