This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Medical, surgical and gynaecological illness complicating pregnancy, Subject name: Gynaecology & Obstetrics
Earliest sign of Mg toxicity: 
Select and explain. 
A. Falling oxygen saturation on pulse oxymetry, B. Respiratory depression, C. Cardiac arrest, D. Anuria
A: Falling oxygen saturation on pulse oxymetry
Answer: A: Falling oxygen saturation on pulse oxymetry 
Explanation: In case of magnesium toxicity ,there can be respirstory depression.In such cases ,infusion should be discontinued,oxygen given and 10ml of 10% calcium gluconate or calcium chloride given IV.Falling oxygen saturation in pulse oxymetry may be an early sign of magnesium toxicity TEXT BOOK OF OBSTETRICS,Sheila Balakrishnan,2nd edition,page no.269