Answer:  Suppurative infundibulitis of undetermined cause

Criteria for diagnosis clinically: Discrete, roundish papules made up mostly of a central, caramel-colored scale-crust, with a hint of rust, but also of an elevated, smooth-surfaced, pink rim are those of a suppurative infundibulitis, the cause of which is not apparent from inspection gross alone.

Differential diagnosis clinically: The lesions fulfill criteria for Kyrle's disease, known originally as "hyperkeratosis follicularis et parafollicularis penetrans," which fundamentally is a suppurative infundibulitis to which infundibular hyperkeratosis in time becomes added.

Criteria for diagnosis histopathologically: Infundibula thinned markedly and widened dramatically by plugs of cornified cells joined by countless neutrophils, those polymorphs also present in large number in the interstitium of the upper half of the dermis, are findings of a suppurative infundibulitis accompanied by striking infundibular hyperkeratosis.

Differential diagnosis histopathologically: The changes pictured are just like those referred to as "perforating folliculitis" as occurs in patients suffering from chronic renal failure and who often are on hemodialysis. That process simply is suppurative infundibulitis that comes to be affiliated with infundibular hyperkeratosis.

Clinicopathologic correlation: The scale is composed of cornified cells and the crust of neutrophils; the caramel color with a tint of rust is the result of the effects combined of suppuration in the epidermis and hemorrhage in the uppermost part of the dermis and in the lower part of the scale-crust; and the rim is elevated by edema in dermal papillae and it is pink because capillaries there are dilated greatly, they, in vivo, having been filled with erythrocytes.

Options for therapy predicated on knowledge of histopathologic findings: Treatment must be directed at the cause presumptive of the suppurative infundibulitis and, in this instance, none could be identified. Because dapsone administered systemically has a muting effect on ingress of neutrophils, it deserves of a try.

1) The term "suppurative infundibulitis" is descriptive of a pattern histopathologic in skin, i.e., suffusion of epidermal infundibula by myriad neutrophils. Those polymorphonuclear leukocytes emerge from venules and capillaries of the superficial plexus and proceed directly to infundibula. Soon, infundibular canals are so laden with neutrophils that the epithelial lining becomes so thin that it ruptures, the mass of neutrophils then being spewed into the dermis where it tends to follow paths in the interstitium. Those stages in the evolution of suppurative infundibulitis are evident in this section of tissue.

2) Every textbook of dermatology, general pathology, and dermatopathology designates the findings histopathologic shown here "suppurative folliculitis" or a variation on that theme, such as "perforating folliculitis." In fact, as is apparent, only the epidermis, in particular, the infundibular part of it, is affected by the process. It is incorrect to characterize those changes as ones of folliculitis when they truly represent an "epidermitis" ("infundibulitis"). The hair follicle consists of a bulb, stem, and isthmus; in continuity with the isthmus is infundibular epidermis that merges imperceptibly with surface epidermis. In short, distinct differences histologic enable a follicle to be distinguished readily from epidermis.

3) Although on grounds morphologic, e.g., clinical and histopathologic, the findings depicted in these photomicrographs can rightly be termed "suppurative infundibulitis," it must be understood that that diagnosis is descriptive of a process, not specific for a disease established clearly. In brief, the term "suppurative infundibulitis" characterizes a particular pattern histopathologic, but it does not convey anything about whether the cause of it is infectious or noninfectious and, if the latter, whether of acne vulgaris, rosacea, pyoderma gangrenosum, Behçet's disease, or idiopathic, to mention but some possibilities. Kyrle's original patients with suppurative infundibulitis, as deemed by him to have "hyperkeratosis follicularis et parafollicularis in cutem penetrans," were diabetics, but identical changes morphologic develop not uncommonly in persons on dialysis consequent to chronic renal failure and on the thighs and legs of men who work beneath hoisted cars and whose dungarees are drenched with machine oils. The inducers of suppurative infundibulitis are many and diverse, from agents occlusive to those responsible for inflammatory bowel disease.

4) Because the countless neutrophils in suppurative infundibulitis manufacture products intensely pruritogenic often to nerve endings in the uppermost part of the dermis, it is no wonder that many a patient, such as one on hemodialysis, rubs vigorously and sometimes scratches excitedly the "primary" lesions, inducing thereby secondary changes of prurigo nodularis or of picker's nodule. Those alterations are evident in features clinical and findings histopathologic that complicate the initial suppurative infundibulitis.

5) Suppurative infundibulitis is analogous to many other distinctive patterns formed by inflammatory processes in skin and subcutaneous fat, among those being urticaria, leukocytoclastic vasculitis, erythema multiforme, pyoderma gangrenosum, erythema nodosum, nodular vasculitis, and even conditions as dissimilar as granuloma annulare and psoriasis. Each of those entities can be identified for what they are morphologically by dint of their singular characteristic pattern, each of them having more than a single cause which, in many instances, is not identifiable either by inspection gross or examination microscopic.