What should a veiled woman suffering from a deficiency in vitamin D and calcium due to lack of sun exposure do, and how should one respond to those who use this as an excuse not to wear the hijab?

A woman must cover her head and body from non-mahram men. The hijab is a firm obligation. The hijab does not prevent the treatment of calcium and vitamin D deficiency, as one can be exposed to the sun in places where there are no non-mahram men, or through windows, or on rooftops. Medical studies have proven that Islamic attire does not contribute to vitamin D deficiency, and that brief daily exposure to sunlight is sufficient to produce an adequate amount of the vitamin.