This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Reproductive physiology and hormones in females, Subject name: Gynaecology & Obstetrics
A 30-year-old G1P0 woman complains of nausea and vomiting for the first 3 months of her pregnancy. She is noted to have a hemoglobin level of 9.0 g/dl and a mean corpuscular volume of 110 fL (normal 90-105 fL). Which of the following is the most likely etiology of the anemia 
Select and explain. 
A. Iron deficiency, B. Vitamin B12 deficiency, C. Folate deficiency, D. Physiologic anemia of pregnancy
C: Folate deficiency
Answer: C: Folate deficiency 
Explanation: This is a macrocytic anemia because the mean corpuscular volume is above normal. Macrocytic anemias include folate deficiency and vitamin B12 deficiency; however, folate deficiency is more commonly seen in pregnancy than vitamin B12 deficiency. Iron deficiency is a microcytic anemia (MCV below normal), and it is the most common cause of anemia in pregnancy. Physiologic anemia of pregnancy is a result of the physiologic hemodilution that occurs in the vasculature. There is a dispropoionate increase in plasma volume over the increased RBC volume, and this "diluted state" also gives the appearance of a fall in the laboratory values of hemoglobin and hematocrit.