Sunscreens absorb ultraviolet radiation before it can be absorbed in the skin. They are recommended to protect the skin from the major toxicities of sun exposure: sunburn and skin cancer. Most available agents primarily absorb UVB, although newer preparations also provide protection against UVA. Physical sunscreens (which are generally opaque, like titanium dioxide and zinc oxide) block all ultraviolet radiation.
The frequency of application of sunscreen is guided by the SPF (sun protection factor) of the preparation. This derived value is the ratio of the time of ultraviolet exposure that causes erythema with the sunscreen to the time that causes erythema without the sunscreen. The higher the SPF, the less frequent the needed application of sunscreen. SPFs of available preparations vary from 2 to 50.
1. Botulinum toxin is used in dermatology to reverse deep wrinkles. Its pharmacological mechanism of action in this use is
(A) Blockade of acetylcholine esterase
(B) Inhibition of release of acetylcholine from motor neurons
(C) Inhibition of synthesis of acetylcholine by inhibiting choline acetyl transferase
(D) Inhibition of acetylcholine binding to mus-carinic receptors
2. Which one of the following agents is known to be a potent teratogen in humans?
(A) Levulan Kerastick
3. Several very useful dermatological agents are derived directly from plants. A compound occurring in the May apple is
(B) Mycophenolate mofetil
4. Melasma is a condition characterized by excess melanin in the epidermis. A topical agent that is frequently successful in this condition is
(D) Botulinum toxin
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