**Core Concept**
Autistic disorder, also known as autism spectrum disorder (ASD), is a neurodevelopmental disorder characterized by impaired social interaction, verbal and nonverbal communication, and repetitive behaviors. Early diagnosis and intervention are crucial for improving outcomes in individuals with ASD.

**Why the Correct Answer is Right**
Intelligence quotient (IQ) is a significant predictor of prognosis in autistic disorder. Individuals with higher IQs tend to have better social and communication skills, which can lead to improved outcomes and a more positive prognosis. This is because higher IQs are associated with better cognitive flexibility, social understanding, and adaptive behaviors. Research has shown that individuals with ASD and higher IQs are more likely to develop age-appropriate language skills, form meaningful relationships, and achieve greater independence.

**Why Each Wrong Option is Incorrect**
* **Option A:** Age of diagnosis is an important factor in determining prognosis, but it is not the most significant predictor. Early diagnosis can lead to earlier intervention, which can improve outcomes, but it does not directly impact prognosis.
* **Option B:** Family history of ASD is a risk factor, but it does not directly influence prognosis. While having a family history may indicate a higher likelihood of developing ASD, it does not predict the severity or outcome of the disorder.
* **Option C:** Presence of co-occurring medical conditions can impact the prognosis of ASD, but it is not the most significant factor. Some medical conditions, such as epilepsy, may be associated with a poorer prognosis, but others, such as attention deficit hyperactivity disorder (ADHD), may have a more neutral impact.

**Clinical Pearl / High-Yield Fact**
Early diagnosis and intervention are crucial for improving outcomes in individuals with ASD. A comprehensive diagnostic evaluation by a multidisciplinary team, including a psychologist, psychiatrist, and speech therapist, can help identify individuals with ASD and develop a personalized treatment plan.

**Correct Answer:** D. Higher IQ. Individuals with higher IQs tend to have a more positive prognosis in autistic disorder due to better social and communication skills, cognitive flexibility, and adaptive behaviors.