**Core Concept**
Tumour lysis syndrome (TLS) is a life-threatening condition that occurs when a large number of cancer cells die rapidly, releasing their contents into the bloodstream. This leads to a complex electrolyte and metabolic derangement.

**Why the Correct Answer is Right**
Hypercalcemia is not a characteristic feature of tumour lysis syndrome. In fact, patients with TLS often develop hypocalcemia due to the high levels of phosphate released from lysed cells, which bind to calcium ions, forming insoluble calcium phosphate complexes. This reduces the level of free calcium in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypernatremia is not typically seen in TLS. Instead, patients often develop hyperkalemia, hyperphosphatemia, and hypocalcemia, leading to metabolic acidosis.

**Option C:** Hyperkalemia is a common feature of TLS. As cancer cells lyse, they release potassium ions into the bloodstream, leading to elevated potassium levels.

**Option D:** Hyperphosphatemia is another characteristic feature of TLS. The rapid release of phosphate from lysed cells can lead to significantly elevated levels, which can contribute to the development of hypocalcemia.

**Clinical Pearl / High-Yield Fact**
In patients with TLS, it is essential to monitor electrolyte levels closely and administer appropriate treatment to prevent cardiac arrhythmias and other complications.

**✓ Correct Answer: B. Hypercalcemia**