Viral Pharyngitis: This is the most likely diagnosis given the symptoms of burning sore throat, headache, and fatigue, which are common in viral infections. The absence of petechiae and a negative HSV DNA test help to narrow down the diagnosis, but many viral pharyngitis cases do not have a specific identifiable cause.

Streptococcal Pharyngitis: Although the absence of petechiae makes this less likely, it's still a consideration, especially if the patient has been exposed to someone with strep throat. The presence of a tiny ulceration could be consistent with this diagnosis.
Infectious Mononucleosis: This condition, caused by Epstein-Barr virus (EBV), can present with sore throat, fatigue, and headache. The presence of a single tiny ulceration could be part of the clinical picture, although typically, there are more systemic symptoms and possibly lymphadenopathy.
Acute Herpangina: This is a viral infection that causes small, painful blisters or ulcers on the roof of the mouth and the back of the throat. The single tiny ulceration on the soft palate could be consistent with herpangina, especially if the patient has been exposed to Coxsackie viruses.

Lemierre's Syndrome: This is a rare but potentially life-threatening condition that starts with a sore throat and can progress to include jugular vein thrombosis and septicemia. Early recognition is crucial, and any signs of severe infection or neck pain should prompt further investigation.
HIV Acute Retroviral Syndrome: In some cases, the initial presentation of HIV infection can include sore throat, fever, and fatigue. Although less common, it's a critical diagnosis not to miss due to its implications for long-term health and the importance of early treatment.
Tuberculosis (TB): TB can present with a variety of symptoms, including chronic sore throat. Although it's less likely given the acute presentation, it's a diagnosis that should be considered, especially in individuals with risk factors for TB exposure.

Behçet's Disease: This is a rare disorder that causes blood vessel inflammation throughout the body and can present with oral ulcers, among other symptoms. The presence of a single ulcer and systemic symptoms like headache and fatigue could potentially fit this diagnosis, but it would be unusual.
Aphthous Ulcers: While common, when they present in an atypical location like the soft palate and are accompanied by systemic symptoms, they could be considered a rare diagnosis in this context.
Malignancy: Although extremely rare, any persistent sore throat or oral ulceration that does not heal should prompt consideration of malignancy, particularly in individuals with risk factors such as smoking or alcohol use.