**Core Concept**
Bence Jones proteinuria refers to the presence of free light chains of immunoglobulins (kappa or lambda) in the urine, which is a hallmark of multiple myeloma or other plasma cell dyscrasias. These light chains are typically produced by neoplastic plasma cells and can be detected in the urine due to their small size and ability to pass through the glomerular filtration barrier.

**Why the Correct Answer is Right**
The correct answer is a monoclonal protein assay, such as the Bence Jones protein assay, which is a sensitive method for detecting these free light chains in the urine. This assay is based on the principle of immunoelectrophoresis, where the light chains are separated from other proteins and detected using specific antibodies. The Bence Jones protein assay is particularly useful for diagnosing multiple myeloma and other plasma cell dyscrasias.

**Why Each Wrong Option is Incorrect**
**Option A:** Urine protein electrophoresis (UPEP) is not the best method for detecting Bence Jones proteinuria, as it is less sensitive than monoclonal protein assays and may not detect small amounts of free light chains. **Option B:** Urine dipstick tests are not specific for Bence Jones proteinuria and may give false-negative or false-positive results. **Option C:** Serum protein electrophoresis (SPEP) is a useful test for detecting monoclonal proteins in the serum, but it is not the best method for detecting Bence Jones proteinuria in the urine.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Bence Jones proteinuria can also be seen in other conditions, such as Waldenström's macroglobulinemia and lymphoma, so a thorough clinical evaluation and additional diagnostic tests are necessary to establish the correct diagnosis.

**Correct Answer:** D.