**Core Concept**
Transillumination is a technique used to assess the presence of fluid or gas within an organ or tissue. It involves shining a light through the organ or tissue, and if it transilluminates (appears to glow), it indicates the presence of a fluid or gas-filled space.

**Why the Correct Answer is Right**
Brilliantly transilluminant swellings are typically indicative of fluid-filled spaces, such as cysts or abscesses. This is because light can easily pass through fluids, causing them to appear bright or "brilliant" when transilluminated. Examples of brilliantly transilluminant swellings include ovarian cysts, lymphangiomas, and abscesses.

**Why Each Wrong Option is Incorrect**
**Option A:** Cystic hygroma is a type of brilliantly transilluminant swelling, often presenting as a soft, fluid-filled mass in the neck. Therefore, this option is incorrect because it is actually an example of a brilliantly transilluminant swelling.
**Option B:** Lipomas are typically non-transilluminant, as they consist of solid fatty tissue. This makes option B incorrect, as lipomas do not exhibit brilliant transillumination.
**Option C:** Hematomas may appear transilluminant if they contain a significant amount of fluid, but they are not as consistently transilluminant as fluid-filled spaces like cysts or abscesses. However, they are not as typically non-transilluminant as option B, so this option could be a trap answer.

**Clinical Pearl / High-Yield Fact**
When performing transillumination, it's essential to use a high-intensity light source and to observe the area from different angles to accurately assess the presence of fluid or gas.

**Correct Answer: D. Lipoma.**