**Core Concept**
Intraventricular haemorrhage (IVH) is a common complication in preterm neonates, characterized by bleeding within the ventricular system of the brain. The underlying risk factors are multifactorial, but a key factor is related to the immaturity of the blood-brain barrier and cerebral vasculature.

**Why the Correct Answer is Right**
Preterm infants with low birth weight are at an increased risk of IVH due to the immaturity of their cerebral vasculature, including fragile capillaries and a lack of myelination. This immaturity leads to increased permeability and susceptibility to bleeding. The most important risk factor is the gestational age, with infants born before 32 weeks of gestation having the highest risk.

**Why Each Wrong Option is Incorrect**
**Option A:** Low Apgar score is a risk factor for various neonatal complications, but it is not the most important risk factor for IVH. While low Apgar scores may indicate fetal distress, they do not directly relate to the risk of IVH.
**Option B:** Maternal hypertension is a risk factor for IVH, but it is not the most important factor. Maternal hypertension can lead to increased blood pressure in the neonate, which may contribute to IVH, but it is not as significant as the gestational age.
**Option C:** Mechanical ventilation is a risk factor for IVH, as it can lead to increased intrathoracic pressure and decreased cerebral perfusion pressure. However, this is not the most important risk factor.

**Clinical Pearl / High-Yield Fact**
Remember that preterm infants with low birth weight are at an increased risk of IVH, and a gestational age of less than 32 weeks is a key risk factor. This is a critical consideration for neonatologists and obstetricians when managing preterm labor and delivery.

**Correct Answer:** C.