**Core Concept**
Transillumination is a clinical technique used to assess the contents of a swelling by shining a light through it. Brilliant transillumination indicates the presence of a fluid-filled or air-filled cavity, whereas non-transillumination suggests a solid mass. This principle is essential in differentiating between various types of swellings.

**Why the Correct Answer is Right**
The correct answer is not listed, but the principle of transillumination helps in diagnosing conditions like a cystic hygroma, which is a fluid-filled sac. This occurs due to the abnormal development of the lymphatic system, resulting in the accumulation of lymphatic fluid. The fluid-filled nature of a cystic hygroma makes it brilliantly transilluminant.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cystic hygroma is indeed brilliantly transilluminant, so this option is not a correct answer.
* **Option B:** Lipomas are solid masses and do not transilluminate, making this option incorrect.
* **Option C:** Abscesses are fluid-filled collections but are not brilliantly transilluminant due to the presence of pus, which scatters light.
* **Option D:** Sebaceous cysts are solid masses with a fluid-filled center, but they do not transilluminate due to the thick walls and variable fluid content.

**Clinical Pearl / High-Yield Fact**
When assessing a swelling, remember that "cystic" implies fluid-filled and brilliantly transilluminant, whereas "abscess" implies a collection of pus that does not transilluminate.

**Correct Answer:** B. Lipomas are solid masses and do not transilluminate.