Which of the following symptoms is a common adverse reaction to first-line TB medications?

First-line TB medications, such as isoniazid, rifampicin, pyrazinamide, and ethambutol, are essential in the treatment of tuberculosis but can lead to various adverse reactions. Hepatitis is one of the well-documented side effects associated with these medications, particularly with isoniazid and rifampicin. The mechanism behind this involves liver enzyme elevation, which can result in inflammation and damage to liver cells, causing symptoms such as jaundice, abdominal pain, or increased liver enzymes detectable in blood tests.

Recognizing hepatitis as a potential adverse effect is crucial for healthcare providers and patients, as monitoring liver function tests during treatment can help in early detection and management of this complication. Understanding the spectrum of potential side effects helps ensure that patients receive safe and effective treatment for their TB infection.

Other symptoms like weight gain, mental confusion, or severe headaches are not commonly associated with the standard first-line TB treatment and are less likely to be expected reactions, making hepatitis the clear answer in this context.