Which of the following conditions may be aggravated by the use of antihistamines?

The correct choice is glaucoma because antihistamines can lead to increased intraocular pressure, which can worsen the condition in individuals already diagnosed with glaucoma. Glaucoma is a group of eye conditions that damage the optic nerve, often related to high intraocular pressure. Antihistamines can cause pupil dilation (mydriasis), which may impede the drainage of aqueous humor in the eye, thus exacerbating the intraocular pressure.

In contrast, while antihistamines can have various effects, conditions like severe headaches or high blood pressure are not directly aggravated by these medications in a consistent manner. Severe headaches may be associated with numerous factors that are not specifically influenced by antihistamines. High blood pressure is not typically considered a direct contraindication for antihistamine use, although individual responses may vary. As for asthma, some antihistamines can even be helpful by reducing allergy symptoms that might trigger asthma attacks, but they do not inherently worsen the condition. Therefore, the correlation is strongest with glaucoma, making it the most appropriate answer.