**Core Concept**
Crigler-Najjar syndrome Type 1 is a rare, severe congenital disorder caused by a complete deficiency of the enzyme uridine diphosphate glucuronosyltransferase (UGT1A1). This enzyme is essential for the conjugation and subsequent excretion of bilirubin, a breakdown product of heme.

**Why the Correct Answer is Right**
In Crigler-Najjar syndrome Type 1, the UGT1A1 enzyme is completely absent or non-functional, leading to a build-up of unconjugated bilirubin in the blood. This results in severe jaundice, neurological damage, and an increased risk of kernicterus, a condition characterized by brain damage due to high bilirubin levels.

**Why Each Wrong Option is Incorrect**
**Option A:** The most common cause of Crigler-Najjar syndrome Type 1 is a point mutation in the UGT1A1 gene, not a deletion. This mutation disrupts the enzyme's activity, leading to the accumulation of bilirubin.
**Option B:** Patients with Crigler-Najjar syndrome Type 1 often present with severe jaundice within the first few days of life, but this option does not specify the underlying cause.
**Option C:** The treatment of choice for Crigler-Najjar syndrome Type 1 is phenobarbital, which induces the expression of the UGT1A1 enzyme, thereby reducing bilirubin levels.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Crigler-Najjar syndrome Type 1 is a rare condition, and it's crucial to recognize the severe consequences of untreated bilirubin toxicity in newborns.

**Correct Answer:** A.