**Question:** A 22-year-old primigravida had a fetal screening ultrasound study at 18 weeks showing a single large cerebral ventricle and fused thalami. On physical examination at birth at 36 weeks' gestation, the infant is small for gestational age and has multiple anomalies, including postaxial polydactyly of hands and feet, cyclopia, microcephaly, cleft lip and palate, and rocker-bottom feet. The infant dies 1 hour after birth. Which of the following CNS abnormalities best explains these findings?

A. Hypoplastic ventricles
B. Dandy-Walker syndrome
C. Hydrocephalus with fused thalami
D. Hypotelorism with holoprosencephaly

**Correct Answer:** C. Hydrocephalus with fused thalami

**Core Concept:**
The given case describes a collection of neurological abnormalities that are commonly seen in holoprosencephaly, a congenital malformation characterized by incomplete separation of the brain hemispheres. In this case, the ultrasound findings of a single large cerebral ventricle and fused thalami are indicative of holoprosencephaly.

**Why the Correct Answer is Right:**
The fetal ultrasound findings of a single large ventricle and fused thalami suggest hydrocephalus and fused thalami, respectively. Hydrocephalus is characterized by an enlarged ventricles due to obstruction or abnormal drainage, which leads to increased intracranial pressure. In this case, the single large ventricle indicates a blockage or obstruction in the ventricular system.

**Why Each Wrong Option is Incorrect:**

A. Hypoplastic ventricles: Hypoplastic ventricles refer to underdevelopment or small size of the ventricles, not a single large ventricle.

B. Dandy-Walker syndrome: Dandy-Walker syndrome is a congenital malformation involving the cerebellum, pons, and medulla oblongata, which is different from the described ultrasound findings.

D. Hypotelorism with holoprosencephaly: Hypotelorism is related to eye abnormalities, while holoprosencephaly is characterized by incomplete separation of the cerebral hemispheres. These are distinct entities and do not fully explain the described ultrasound findings.

**Clinical Pearl:**
Holoprosencephaly is a critical malformation syndrome that should be considered in differential diagnosis when faced with an ultrasound showing a single large ventricle and fused thalami. These features are indicative of holoprosencephaly, which should prompt further workup for chromosomal abnormalities and consultation with a pediatric neurologist or geneticist.