**Core Concept**
Menorrhagia is a condition characterized by heavy menstrual bleeding, which can lead to significant blood loss and discomfort for women. The definition of menorrhagia is often based on the amount of menstrual blood loss, which can be quantified using various methods.

**Why the Correct Answer is Right**
The correct answer is based on the definition of menorrhagia provided by the American College of Obstetricians and Gynecologists (ACOG). According to ACOG, menorrhagia is defined as menstrual blood loss of more than 80 mL per cycle. This definition takes into account the average menstrual blood loss of a healthy woman, which is around 30-40 mL per cycle. A blood loss of more than 80 mL per cycle is considered excessive and may lead to anemia, fatigue, and other complications.

**Why Each Wrong Option is Incorrect**
**Option A:** 40 mL - This amount is within the normal range of menstrual blood loss and would not be considered menorrhagia.
**Option B:** 60 mL - While this amount is higher than normal, it is still below the threshold of 80 mL defined by ACOG as menorrhagia.
**Option C:** 120 mL - This amount is indeed excessive, but it is not the correct answer because it is more than the defined threshold of 80 mL.

**Clinical Pearl / High-Yield Fact**
It's essential to note that menorrhagia can be a symptom of underlying conditions such as uterine fibroids, endometriosis, or hormonal imbalances. A thorough evaluation by a healthcare provider is necessary to determine the underlying cause of heavy menstrual bleeding.

**Correct Answer: D. 80 mL**