**Core Concept**
Magnesium sulphate toxicity is a serious condition that can occur when excessive amounts of magnesium are absorbed by the body. This can lead to a range of systemic effects due to the ion's role in neuromuscular transmission and cardiac conduction. In gynaecology and obstetrics, magnesium sulphate is commonly used for eclampsia prophylaxis and treatment.

**Why the Correct Answer is Right**
Magnesium sulphate toxicity typically presents with neuromuscular blockade, including loss of deep tendon reflexes such as the patellar reflex. This is due to the inhibition of acetylcholine release at the neuromuscular junction, which disrupts normal muscle contraction. Respiratory depression is also a common manifestation, as magnesium sulphate can depress the central nervous system and reduce respiratory drive. Cardiac arrest can occur in severe cases due to the ion's effects on cardiac conduction. In contrast, oliguria (decreased urine output) is not typically associated with magnesium sulphate toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** Loss of patellar reflex is a hallmark of magnesium sulphate toxicity, making this option incorrect. The patellar reflex is a common indicator of neuromuscular blockade.
**Option C:** Respiratory depression is a well-documented effect of magnesium sulphate toxicity, making this option incorrect.
**Option D:** Cardiac arrest can occur in severe cases of magnesium sulphate toxicity, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
In the setting of magnesium sulphate toxicity, the patellar reflex can be used as a quick indicator of neuromuscular blockade. A decreased or absent patellar reflex may prompt further investigation and management to prevent progression to more severe complications.

**Correct Answer:** ✓ Correct Answer: B. Oliguria.