A 1 1/2 year old female child is brought to you with history of delay in developmental milestones. The child was apparently normal till 5 months of age following which the milestones became delayed. Now some of the milestones attained have regressed. On examination the head circumference is less than expected. The child has also got impaired coordination. What is your diagnosis?

**Core Concept**
Early infantile autism, also known as classic autism, is a neurodevelopmental disorder characterized by impaired social interaction, communication, and coordination. It often presents with delayed or regressed developmental milestones, particularly in language and motor skills.

**Why the Correct Answer is Right**
The child's symptoms of delayed development, regressed milestones, and impaired coordination are indicative of early infantile autism. The decreased head circumference, also known as microcephaly, is a common feature in children with autism. The impaired coordination is likely due to difficulties in motor planning and execution, which are hallmarks of autism. The child's age and developmental history, including normal development until 5 months followed by regression, are also consistent with autism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe the child's symptoms. The child's delayed development and regressed milestones are not typical of cerebral palsy, which usually presents with spasticity, weakness, or paralysis.

**Option B:** This option is incorrect as it does not account for the child's impaired coordination, which is a key feature of autism. The child's developmental delay and regressed milestones also do not suggest a diagnosis of Down syndrome, which typically presents with intellectual disability, characteristic facial features, and other physical anomalies.

**Option C:** This option is incorrect as it does not accurately describe the child's symptoms. The child's impaired coordination and regressed milestones are not typical of attention deficit hyperactivity disorder (ADHD), which usually presents with inattention, hyperactivity, and impulsivity.

**Option D:** This option is incorrect as it does not account for the child's impaired coordination and regressed milestones. The child's developmental delay and decreased head circumference are not typical of fragile X syndrome, which typically presents with intellectual disability, characteristic facial features, and other physical anomalies.

**Clinical Pearl / High-Yield Fact**
Autism spectrum disorder (ASD) is a heterogeneous condition, and early recognition is crucial for timely intervention and improved outcomes. The American Academy of Pediatrics recommends screening for ASD in children at 18 and 24 months of age.

**Correct Answer:** D.

✓ Correct Answer: C. Rett disorder