**Core Concept**
The bulge sign is a clinical indicator used to assess the amount of joint effusion in the knee joint. It is based on the principle that a certain volume of fluid accumulation can cause a visible bulge in the joint space. This sign is crucial in diagnosing joint disorders and guiding further management.

**Why the Correct Answer is Right**
The bulge sign is typically seen with a small amount of fluid accumulation, specifically less than 30 ml. This is because even a small volume of fluid can cause a noticeable bulge in the joint space. The joint capsule is a thin, fibrous structure that encloses the synovial cavity, and when fluid accumulates, it stretches the capsule, leading to a visible bulge. This is a sensitive indicator of joint effusion, making it a valuable clinical tool.

**Why Each Wrong Option is Incorrect**
**Option A:** 100 ml is an excessive amount of fluid, and a bulge sign is not typically seen at this volume. In fact, a large joint effusion would likely cause significant discomfort and swelling.
**Option B:** 400 ml is an even larger amount of fluid, and a bulge sign would not be the primary concern at this stage. The joint would likely be significantly swollen and painful.
**Option C:** 200 ml is still a substantial amount of fluid, and a bulge sign might not be the most reliable indicator at this volume.

**Clinical Pearl / High-Yield Fact**
Remember that the bulge sign is a sensitive indicator of joint effusion, but it can be absent even with significant fluid accumulation. Therefore, it is essential to combine clinical examination with imaging studies to confirm the diagnosis.

**✓ Correct Answer: D. < 30 ml**