**Core Concept**
Tumour lysis syndrome (TLS) is a potentially fatal complication of cancer treatment, characterized by the rapid release of intracellular components into the bloodstream, leading to electrolyte imbalances and acute kidney injury. This condition is particularly common in lymphoreticular malignancies, such as leukemia and lymphoma.

**Why the Correct Answer is Right**
The key laboratory investigations required to diagnose TLS include monitoring of renal function (urea and creatinine) and assessment of electrolyte imbalances that can lead to cardiac arrhythmias and other complications. Phosphate levels are critical, as hyperphosphatemia can lead to hypocalcemia, which in turn can cause cardiac arrhythmias. Additionally, monitoring of potassium levels is essential, as hyperkalemia can lead to cardiac arrest. Calcium levels are also important to monitor, as hypocalcemia can occur due to phosphate binding to calcium.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because chloride levels are not directly related to the diagnosis of TLS. While chloride levels may be affected indirectly due to changes in renal function, they are not a critical parameter in the diagnosis of TLS.
**Option C:** This option is incorrect because magnesium levels are not typically monitored in the context of TLS. While magnesium imbalance can occur in various clinical scenarios, it is not a key parameter in the diagnosis of TLS.
**Option D:** This option is incorrect because sodium levels are not directly related to the diagnosis of TLS. While sodium levels may be affected indirectly due to changes in renal function, they are not a critical parameter in the diagnosis of TLS.

**Clinical Pearl / High-Yield Fact**
In patients at risk of TLS, it is essential to monitor electrolyte levels closely, particularly phosphate, calcium, and potassium, and to adjust treatment accordingly to prevent cardiac complications.

**✓ Correct Answer: B. Urea, creatinine, phosphate, Ca2+ and K+**