Primary amenorrhea is defined as the failure of menarche onset by age?

Primary amenorrhea is defined as the absence of menstrual periods by a certain age in females, which can vary slightly based on different clinical guidelines. The most widely accepted definition is the failure to achieve menarche, which is the first occurrence of menstruation, by age 13 in the absence of secondary sexual characteristics.

The age of 13 is significant because it is typically expected that girls will begin to menstruate within a few years after developing secondary sexual characteristics, such as breast development or pubic hair. If these characteristics are not present by this age, it raises concerns about underlying conditions that may be causing a delay in menarche, such as hormonal imbalances, genetic conditions, or anatomical abnormalities.

While other ages are also used in different contexts (such as 15 or 17 years) as markers of primary amenorrhea, the age of 13 is commonly recognized for its clinical significance in evaluating and managing girls who have not menstruated by this age. Thus, the correct choice aligns with the established definitions used in medical practice and literature.