Donald Trump was spotted in public with a nasty new rash on his neck, and one clinical immunologist believes a pre-cancerous lesion could potentially be the culprit.The 79-year-old president on Monday awarded the Medal of Honor at the White House to Vietnam veteran Terry P. Richardson, who is credited with saving the lives of 85 soldiers during a 1968 battle, and eagle-eyed observers noted that photos showed a heavy rash on the right side of Trump's neck at the event."Whoa -- this is new," independent journalist Aaron Rupar posted on his social media, showing a photo credited to Getty photographer Saul Loeb. "Trump has a significant rash-like injury on his neck today in addition to his disfigured hand."The White House responded to rumors about the rash, insisting that it was caused by a "preventative" cream that is commonly used. Trump's doctor said it would be gone in a week.Some on the internet accused the White House doctor of lying, but one doctor had a different suggestion. He noted that there are numerous possibilities for the source of the rash, including contact dermatitis and shingles, but specifically noted a "pre-cancerous lesion" caused by aging and sun damage works best with the clues."I have been tagged in a lot of videos," said Dr. Zachary Rubin, who posted the videos for educational purposes and not as a literal diagnosis of the president. He first noted that you can't diagnose just by a picture like he's seen, but said, "From an allergy perspective, if you have a rash that is blistering, red, very itchy, and it's in one particular spot... there's a possibility of something called contact dermatitis."Noting it was just one example, he added that a "red blistering rash" could be shingles - the re-activation of chicken pox during stress. He also noted that insect bites could potentially be responsible, as well.But in a follow-up video, the doctor offered an update to the potential differential diagnosis for Trump's rash, based on information provided by the White House.Noting that the White House claimed it was from a "preventive" cream, the doctor added some extra information. In addition to contact dermatitis and shingles, he said it could be "something called actinic keratosis - that is a pre-cancerous lesion that is the result of aging as well as sun damage."According to the doctor, the cream used to treat that is 5-FU, which "can cause the skin to look very irritated and red."If this pre-cancerous lesion is identified, "it tends to be treated," according to the doctor, who noted it can lead to a type of skin cancer called Squamous cell carcinoma.