I remember being told in veterinary school that “common things happen commonly” and, “If you hear hoof beats, don’t think: zebras.” In other words, lupus is a relatively rare disease, so don’t be too worried yet. There are other, more common, things that could cause the symptom you are seeing.
If there is a singular spot and this is the only symptom you notice in your dog, it could simply be a scratch or wound. When the skin is damaged and new skin cells grow in to repair that wound, the new skin is often paler than the surrounding tissue. Sometimes, wounds on the nose take longer to heal because the dog is able to lick them frequently. This can delay healing in any wound.
The spot could also be due to sunburn. This is especially common in dogs with lighter skin tones and fur colors. Another possibility would be a bacterial skin infection that can cause depigmentation and scabs on the nose. Vitiligo is yet another skin disorder that can cause depigmentation of the skin, but usually it would involve more than just a small single spot. Most of the other possibilities would involve multiple symptoms, not just a single lesion on the nose.
To help your veterinarian rule out various disorders, it is important first to determine whether the change occurred on the haired part of your dog’s nose skin or the tip of the nose (planum nasale) where there is no fur.
The next step toward a diagnosis would be for your vet to perform various tests. A skin scraping can help to rule out parasitic causes. Cytology, such as a fine- needle aspiration or an impression smear, can rule out bacterial or yeast infections and possibly neoplasia (an abnormal proliferation of cells). To rule out ringworm, a fungal skin infection that can also cause depigmentation of one area of the skin, a fungal culture could be performed.
If all of those tests are negative, a skin biopsy would be indicated to diagnose lupus or another autoimmune disease.
Since you asked about lupus, there are two types: systemic and discoid. Systemic means lupus throughout the entire body, so other organ systems besides your dog’s skin would be involved. Discoid lupus mostly affects just the skin, and is a mild, manageable disease that sometimes goes away on its own. It is often made worse by the dog being in the sun too much. Females are slightly more affected with discoid lupus than males. A biopsy would be needed to diagnose discoid lupus.
Any breed of dog can have lupus, but some breeds, such as Collies, have a recognized increased frequency. Pit Bulls are not one of those breeds.