This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
All of the statements regarding progesterone only pill are true, except 
Select and explain. 
A. Inhibits ovulation, B. Failure rate is the same as combined OCP, C. Irregular bleeding is a known complication, D. Acts by altering the cervical mucous
B: Failure rate is the same as combined OCP
Answer: B: Failure rate is the same as combined OCP 
Explanation: Failure rate of combined OCP is 0.2/100 women years, whereas failure rate of progesterone only pill is about 0.5 - 2 per 100 women years of use. Progesterone only pill acts by thickening cervical mucous, causing atrophy of endometrium or by inhibiting ovulation.