Which organism is responsible for causing painful chancroid?

Haemophilus ducreyi is the organism responsible for causing chancroid, which is characterized by painful genital ulcers. This bacterium is a fastidious, gram-negative coccobacillus that primarily infects the mucous membranes of the genital area. The ulcers associated with chancroid are typically painful and often accompanied by swollen lymph nodes, which can lead to complications if untreated.

In contrast to other sexually transmitted infections, the ulcers from chancroid are distinct in that they tend to be more painful and are often irregular in shape. This feature, along with the specific clinical presentation, helps differentiate chancroid from syphilis (caused by Treponema pallidum, which causes painless ulcers) and infections like those caused by Neisseria gonorrhoeae or Chlamydia trachomatis. These latter infections can cause urethritis and cervicitis but do not typically present with the same characteristic ulcerations seen in chancroid.