The patient: A 68-year-old woman in the U.S.

The symptoms: Over the course of six weeks, dark patches appeared on the woman's arms and legs. The splotches ranged from a bruise-like dark blue and purple to jet-black.

She also told doctors that, two weeks before the patches appeared, she'd started a course of minocycline, an oral antibiotic.

She had been prescribed a daily 100-milligram dose of the drug to treat symptoms of rosacea, which causes chronic inflammation and redness of the face. This inflammation can drive the formation of small, red bumps and pus-filled pimples on the skin, and evidence suggests that antibiotics like minocycline can help eliminate those bumps. In addition to killing bacteria, the drug is known to have anti-inflammatory effects, the latter of which may best explain how it treats rosacea.

The diagnosis: A well-established side effect of minocycline is hyperpigmentation, in which patches of skin become darker than the skin surrounding them. In this case, the woman was diagnosed with type II minocycline-induced hyperpigmentation, which is "defined by blue-gray discoloration of normal skin on the extensor surfaces of the arms and legs," according to a report of the case.

The antibiotic can also trigger two other types of hyperpigmentation. Type I shows up as blue-black discoloration on scarred or inflamed skin on the face, rather than on healthy skin on the limbs, and type III appears as muddy-brown pigmentation on sun-exposed areas of the body.

The treatment: Doctors advised the patient to stop taking minocycline and to avoid sun exposure, as ultraviolet light is thought to worsen hyperpigmentation in these cases. Six months later, the hyperpigmentation in her limbs had "abated somewhat" but was still visible.

What makes the case unique: Hyperpigmentation is not necessarily uncommon for minocycline users with rosacea; one frequently cited study estimates that the side effect shows up in about 28% of people in this population, although that research included only a small number of patients. The true incidence of the side effect is unclear.

This woman's case was notable, in part, because of how quickly the discoloration developed ‪—‬ within two weeks of starting minocycline. While type I minocycline-induced hyperpigmentation can show up soon after a person starts taking the drug, type II and type III usually take longer to appear.

"It typically develops after months of treatment but may rarely occur with shorter courses," the case report authors noted. Type II and type III are thought to be dose-dependent, in that a certain amount of the drug needs to build up in the body before the dark patches show up.

The cause of minocycline-induced hyperpigmentation isn't fully understood, but it's thought to result from how the antibiotic gets broken down in the body. The metabolites, or byproducts, created in this process bind to iron and are then picked up by immune cells called macrophages, in which they accumulate. Additionally, the drug may drive up the activity of cells that make melanin — the pigment that gives skin its color — while also binding to melanin, thereby creating complexes of dark pigment that linger in the skin.

Once a person stops taking minocycline, the pigmentation can take months to years to dissipate, reports suggest. In type III cases, it sometimes never goes away.

For more intriguing medical cases, check out our Diagnostic Dilemma archives.

Can you guess the diagnosis in these strange medical cases? Find out with our diagnostic dilemma quiz!