Final answer:
For an 18-year-old with brittle diabetes mellitus and severe hypertension, beta-blockers are a bad choice of antihypertensive medication due to their potential to mask hypoglycemia symptoms and negatively affect lipid metabolism and insulin sensitivity.
Step-by-step explanation:
An 18-year-old with brittle diabetes mellitus and hypertension of 190/110 should avoid certain antihypertensives that could exacerbate their condition. A poor choice of antihypertensive in this case would be a beta-blocker. Beta-blockers are not recommended because they can mask the symptoms of hypoglycemia, a critical issue for someone with brittle diabetes. Furthermore, beta-blockers may have adverse effects on lipid metabolism and insulin sensitivity, which can be especially problematic in individuals with diabetes.