**Core Concept**
Microcephaly refers to a condition where an infant's head circumference is smaller than two standard deviations below the mean for their age and sex, typically defined as a head circumference less than 33 cm at birth. This condition can result from various prenatal, perinatal, or postnatal factors that affect brain development.

**Why the Correct Answer is Right**
Alexander disease is a rare, degenerative disorder classified as a leukodystrophy, characterized by the accumulation of glial fibrillary acidic protein (GFAP) in astrocytes. It typically presents with macrocephaly (enlarged head size) due to excessive brain tissue and not microcephaly. The other options can cause microcephaly through different mechanisms. Edward syndrome (Trisomy 18) often presents with microcephaly due to severe growth restriction and congenital anomalies. Maternal smoking during pregnancy can lead to intrauterine growth restriction (IUGR), resulting in microcephaly. Congenital Toxoplasmosis can cause microcephaly by inducing an inflammatory response and apoptosis in the developing brain.

**Why Each Wrong Option is Incorrect**
**Option A:** Edward syndrome is known for causing microcephaly due to the effects of Trisomy 18 on fetal development.
**Option B:** Maternal smoking contributes to intrauterine growth restriction (IUGR), which can result in microcephaly.
**Option D:** Congenital Toxoplasmosis can cause microcephaly through an inflammatory response and apoptosis in the developing brain.

**Clinical Pearl / High-Yield Fact**
When assessing an infant with microcephaly, it is essential to consider a comprehensive differential diagnosis, including genetic disorders like Edward syndrome, congenital infections like Toxoplasmosis, and environmental factors such as maternal smoking.

✓ Correct Answer: C. Alexander disease