**Core Concept**
Trichomonas vaginalis is a sexually transmitted protozoan infection that causes a characteristic vaginal discharge in infected women. The discharge is often associated with an unpleasant odor and is a key diagnostic feature of the infection.

**Why the Correct Answer is Right**
The greenish discharge in trichomonas vaginalis infection is due to the presence of a high number of protozoa and their breakdown products in the vaginal secretions. The discharge is often described as frothy, yellow-green or greenish in color and has a strong, foul odor. This is because the protozoa feed on the vaginal epithelial cells, leading to an overgrowth of bacteria and an increase in the production of volatile sulfur compounds, which contribute to the characteristic odor of the discharge.

**Why Each Wrong Option is Incorrect**
**Option B:** White curdy discharge is more characteristic of candidal vaginitis, which is a fungal infection caused by Candida albicans. The discharge in candidal vaginitis is often described as thick, white, and cottage cheese-like.
**Option C:** Blood-stained discharge can be seen in various gynecologic conditions, including menstrual disorders, cervical cancer, and ectopic pregnancy. However, it is not a characteristic feature of trichomonas vaginalis infection.
**Option D:** Milky discharge is often seen in cases of bacterial vaginosis, which is caused by an overgrowth of anaerobic bacteria in the vagina.

**Clinical Pearl / High-Yield Fact**
It's essential to note that trichomonas vaginalis infection is often asymptomatic, and the diagnosis is often made based on the presence of characteristic discharge and a history of recent sexual contact. A high index of suspicion is required to diagnose this infection, particularly in women who are sexually active.

**✓ Correct Answer: A. Greenish discharge**