Viral Pharyngitis: This is the most common cause of sore throat in adults, and the presence of a red throat with white pimples (which could be indicative of viral exanthems or reactive lymphoid hyperplasia) along with a negative rapid strep test supports this diagnosis. Viral pharyngitis can be caused by a variety of viruses, including rhinovirus, coronavirus, adenovirus, and others.

Streptococcal Pharyngitis (despite negative rapid test): Although the rapid strep test is negative, it is not 100% sensitive. Therefore, if clinical suspicion remains high (based on symptoms like fever, swollen and tender lymph nodes, absence of cough, and the appearance of the throat), a throat culture might be necessary to rule out streptococcal infection.
Infectious Mononucleosis: Caused by Epstein-Barr virus (EBV), this condition can present with sore throat, fever, lymphadenopathy, and sometimes a rash or exudative pharyngitis. It's more common in younger populations but can occur at any age.
Acute Laryngitis or Tracheitis: These conditions can cause a sore throat and are usually viral in nature. They might present with hoarseness or a cough, respectively.

Lemierre's Syndrome: A rare but potentially life-threatening condition that starts with pharyngitis, followed by neck pain and fever, and can progress to septicemia and thrombophlebitis of the internal jugular vein. It's caused by Fusobacterium necrophorum.
Epiglottitis: Although more common in children, epiglottitis can occur in adults, especially those with compromised immune systems. It presents with severe sore throat, difficulty swallowing, shortness of breath, and can lead to airway obstruction.
Diphtheria: A bacterial infection caused by Corynebacterium diphtheriae, characterized by a membrane covering the tonsils and throat, which can cause respiratory and cardiac complications if not treated promptly.

Gonococcal Pharyngitis: Caused by Neisseria gonorrhoeae, this condition is more common in sexually active individuals and can present with sore throat, often without other symptoms.
Tuberculous Pharyngitis: A rare form of tuberculosis that affects the pharynx, presenting with chronic sore throat, weight loss, and other systemic symptoms.
Syphilitic Pharyngitis: Caused by Treponema pallidum, this condition can occur in the primary stage of syphilis and presents with a painless sore throat and other mucocutaneous lesions.