A sexually active 18-year-old presents with postcoital spotting, dysuria, and a yellow discharge. What is the most likely diagnosis?

The clinical presentation described—postcoital spotting, dysuria, and yellow discharge—strongly suggests a diagnosis of chlamydia. Chlamydia trachomatis is a common sexually transmitted infection (STI) that often affects young sexually active individuals, particularly women. Symptoms can include abnormal vaginal discharge, bleeding between periods or after intercourse (postcoital spotting), and painful urination (dysuria).

Given the age of the patient and the symptoms, chlamydia is particularly relevant because it can lead to complications such as pelvic inflammatory disease if left untreated. The yellow discharge aligns with the typical presentation of chlamydial infections, which may be purulent.

In contrast, cervical cancer typically presents with irregular bleeding, but the age and symptom profile here are more indicative of an infection rather than malignancy. A tampon injury would usually not present with symptoms such as a yellow discharge and is less plausible considering the specific context of sexual activity and the other symptoms. Primary syphilis generally presents with a painless ulcer (chancre) at the site of infection, along with systemic symptoms that are not indicated in this case.

Thus, the combination of the listed symptoms makes chlamydia the most likely diagnosis.