What additional resistance characterizes extremely drug-resistant tuberculosis?

The correct answer highlights that extremely drug-resistant tuberculosis (XDR-TB) is characterized by resistance to at least one second-line injectable drug. In the context of tuberculosis treatment, XDR-TB is a severe form of multidrug-resistant tuberculosis (MDR-TB).

MDR-TB is already resistant to at least isoniazid and rifampicin, the two most effective first-line antibiotics. However, what defines XDR-TB is its additional resistance, which includes the inability to treat the infection with the available second-line treatments. These second-line drugs include fluoroquinolones and injectable medications such as amikacin, kanamycin, and capreomycin. Resistance to at least one of these second-line injectables indicates a significant challenge in treatment options, making it a critical aspect of XDR-TB.

Resistance solely to Isoniazid or fluoroquinolones does not encompass the full spectrum of resistance seen in XDR-TB. Furthermore, resistance to all first-line and second-line drugs would classify a case as pan-drug resistant (PDR-TB), which is even more severe and does not precisely define the characteristic of XDR-TB. Thus, the additional resistance to at least one