**Core Concept**
The Nugent score is a scoring system used to assess the bacterial vaginosis (BV) in a vaginal smear. It is a semi-quantitative scoring system that helps in evaluating the presence and quantity of clue cells, bacteria, and other characteristics of a vaginal smear.

**Why the Correct Answer is Right**
The Nugent score includes several parameters such as the presence and quantity of lactobacilli, clue cells, gram-variable rods, and gram-negative rods. It also assesses the presence of white blood cells, epithelial cells, and the overall consistency of the vaginal smear. The score ranges from 0 to 10, with higher scores indicating a higher likelihood of BV.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Nugent score does not include the pH level of the vaginal discharge. The pH level is an important parameter in diagnosing BV, but it is not included in the Nugent score.

**Option B:** This option is incorrect because the Nugent score does not include the presence of yeast. While yeast infections can cause vaginal discharge, they are not evaluated using the Nugent score.

**Option C:** This option is incorrect because the Nugent score does not include the presence of trichomonas. Trichomonas is a protozoan parasite that can cause vaginitis, but it is not evaluated using the Nugent score.

**Clinical Pearl / High-Yield Fact**
The Nugent score is a useful tool in diagnosing bacterial vaginosis, but it should be used in conjunction with clinical symptoms and other diagnostic tests to confirm the diagnosis.

**Correct Answer: D. Nugent score includes all except pH level of vaginal discharge.**