## Core Concept
The condition described for the 6-month-old child—characterized by a port-wine stain, mental retardation, and recurrent focal seizures—is suggestive of **Sturge-Weber syndrome**. This syndrome is a congenital disorder that affects the development of certain blood vessels, leading to abnormalities in the brain, skin, and eyes.

## Why the Correct Answer is Right
Sturge-Weber syndrome is associated with a **port-wine stain** (a birthmark) typically on the face, often in the distribution of the trigeminal nerve, **mental retardation** or developmental delays, and **seizures**. The condition results from a mutation in the GNA11 gene, leading to abnormal development of blood vessels. The port-wine stain is a result of a vascular malformation, and the neurological symptoms arise from calcifications and other vascular abnormalities in the brain.

## Why Each Wrong Option is Incorrect
- **Option A:** Typically, Sturge-Weber syndrome involves **leptomeningeal angiomatosis**, which can lead to seizures, often focal. So, seizures are indeed associated with the condition.
- **Option B:** The condition is known for causing **developmental delays** and **intellectual disability** (mental retardation), making this statement true.
- **Option C:** A **port-wine stain** is a hallmark of Sturge-Weber syndrome, usually present at birth or shortly thereafter.

## Clinical Pearl / High-Yield Fact
A key clinical feature to remember about Sturge-Weber syndrome is the **port-wine stain**, which often follows the distribution of the trigeminal nerve. Additionally, **ocular involvement**, such as glaucoma, can occur, particularly if the port-wine stain involves the eyelid. Early recognition of the syndrome can help in managing its complications.

## Correct Answer Line
**Correct Answer: D.**