A 19-year-old woman has vulvar itching and a yellow-gray discharge with a pH of 6. What is the most likely cause?

The symptoms described in the scenario—vulvar itching, yellow-gray discharge, and an elevated vaginal pH of 6—are characteristic of bacterial vaginosis. Bacterial vaginosis is caused by an imbalance in the normal flora of the vagina, typically involving a decrease in beneficial Lactobacillus species, which normally help to maintain a low pH (around 4-4.5) in the vagina. When Lactobacillus levels drop, the pH increases, which can lead to symptoms including a thin, grayish discharge that may have a fishy odor, along with itching.

In this case, the elevated pH of 6 highlights the likelihood of bacterial vaginosis rather than candidiasis or trichomoniasis. Candidiasis usually presents with a lower pH and a thicker, white discharge, while trichomoniasis often results in a frothy discharge and is associated with a pH greater than 4.5 but can also present with more distinct symptoms of irritation. Herpes simplex virus would typically cause painful lesions rather than discharge with this particular presentation.

Given the specific combination of symptoms and the elevated pH, bacterial vaginosis is the most plausible cause in this clinical scenario.