(8) Pustule. A pustule (figure 3-9) is a pus-filled lesion that can result from
infection of vesicles or bullae. The pus is a thick fluid composed primarily of dead white
blood cells (leukocytes) and a thin liquid and often the microbiologic agent responsible
for the inflammation. The pustule may appear white, yellow, or greenish-yellow,
depending on the color of the material being deposited in the tissue. An example of a
common pustule is a follicular or hair pustule. Hair pustules are usually shaped like a
cone, have a hair in the center, and heal without scarring.
Figure 3-9. Pustule.
d. Secondary Skin Lesions. Secondary lesions are the result of some primary
lesion. A number of conditions such as crust and scale are characteristic of secondary
(1) Crust. The dried residue of fluid, blood, or pus on an area of lost or
damaged skin surface is crust (figure 3-10). Crusts may be thin, delicate, and easily
torn or damaged or the crust may be thick and sticking to the skin. Crusts formed from
dried serum (the clear fluid of blood) are yellow while crust formed from pus are yellow-
green, and crusts formed from blood are dark red or brown. When dealing with crust, it
is most important to determine what is underneath the crust--dermatitis, superficial
infection, or ulcer.
Figure 3-10. Crust.