A 57-year-old woman presents with a sore throat and fever. What is likely causing her condition?

The choice of Group A streptococcus as the likely cause of the patient's condition is supported by the common presentation of a sore throat and fever, which are often associated with strep throat, an infection caused by this bacterium. Group A streptococcus, also known as Streptococcus pyogenes, is a predominant pathogen in cases of pharyngitis, especially in adults, and is characterized by symptoms including severe throat pain, fever, and sometimes the presence of white patches on the tonsils.

While viral infections can also lead to sore throat and fever, they typically present with additional symptoms such as cough, runny nose, or laryngitis and may resolve without the need for antibiotic treatment. Glandular fever, which is caused by the Epstein-Barr virus, usually presents with significant fatigue, swollen lymph nodes, and is more common in younger individuals. Thyroiditis, on the other hand, typically manifests with neck pain, swelling, and, depending on the type, may cause changes in thyroid hormone levels but is less commonly associated with acute throat pain and fever.

Thus, given the age of the patient and the classic symptoms, Group A streptococcus emerges as a very strong contender for the cause of her condition.