This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Contraception, Subject name: Gynaecology & Obstetrics
Which natural family planning method is based on Ogino Knauss theory ? 
Select and explain. 
A. BBT method, B. Rhythm method, C. Lactational aneronhea, D. Withdrawal method
B: Rhythm method
Answer: B: Rhythm method 
Explanation: In Knaus-Ogino method, the feile period is determined by subtracting 18 days from the shoest cycle and 10 days from the longest cycle which gives the first and the last day of feile period, respectively. This method will result in approximately 25 pregnancies per 100 woman years. The failure results from irregular ovulation or from irregular menstrual cycles. Some couples prefer this method on religious grounds or because they find other methods unacceptable. Reference: Shaw's Textbook of Gynaecology, 16th edition,page 265.