**Core Concept**
Tumour lysis syndrome (TLS) is a life-threatening condition that occurs when large numbers of cancer cells die rapidly, releasing their contents into the bloodstream. This leads to an imbalance of electrolytes and metabolites, which can cause serious complications.

**Why the Correct Answer is Right**
In TLS, the rapid lysis of cancer cells releases calcium from the lysed cells into the bloodstream. This results in **hypercalcemia**. The correct answer, **B. Hypercalcemia**, is a key feature of TLS, which is often overlooked in the context of other metabolic disturbances.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyperuricemia is indeed a feature of TLS, as the breakdown of nucleic acids from lysed cells leads to the production of uric acid. However, it is not the correct answer in this context.
**Option C:** Hyperphosphatemia is another common finding in TLS, as the lysed cells release phosphate into the bloodstream. Like hyperuricemia, it is a consequence of TLS, but not the correct answer here.
**Option D:** Hypocalcemia is incorrect because, as mentioned, TLS typically results in hypercalcemia due to the release of calcium from lysed cells.

**Clinical Pearl / High-Yield Fact**
When encountering a patient with TLS, it is essential to remember that the condition often presents with a complex interplay of electrolyte imbalances. A key clinical correlation is that TLS often occurs in patients with high cell turnover rates, such as those with acute leukemia or lymphoma.

**✓ Correct Answer: B. Hypercalcemia**