Question

Which of the following is not seen in humans?

• A. Estrous cycle
• B. Menstrual cycle
• C. Endometrial cycle
• D. Ovarian cycle

Hint:

Humans menstruate; they do not have a heat or estrus period.

Answer 1

The Correct Option is A

Step 1: The question asks which reproductive cycle is absent in humans.
Step 2: The estrous cycle is the reproductive cycle of most female mammals (rats, dogs, cows, etc.). In an estrous cycle the female is sexually receptive only during a defined period called estrus (heat), and if fertilisation does not occur the thickened endometrium is reabsorbed rather than shed.
Step 3: Humans and other higher primates instead have a menstrual cycle. Here the endometrium that is not used for implantation is shed as menstrual bleeding, and the female can be receptive throughout the cycle.
Step 4: The menstrual cycle is the overall cycle in humans, and it has component cycles described together: the ovarian cycle (follicular and luteal phases in the ovary) and the endometrial (uterine) cycle (proliferative, secretory and menstrual phases of the endometrium). All three are seen in humans.
Step 5: Therefore the cycle not seen in humans is the estrous cycle, while the menstrual, endometrial and ovarian cycles are all present.
Answer: Option A (Estrous cycle).