A 24-year-old woman presents to your Emergency Department with a five-day history of sore throat, mild cough, and discomfort on swallowing.

 She denies fever, rash, or coryzal symptoms.

 She was treated with penicillin V by her GP three days ago with no improvement.

She mentions a new sexual partner and reports unprotected oral intercourse last week.

 She denies vaginal discharge or pelvic pain.

On examination, her temperature is 37.6 °C, HR 90 bpm, and BP 120/72 mmHg.

 The tonsils and posterior pharynx are erythematous without exudate or vesicles.

 No cervical lymphadenopathy or pseudomembrane is seen.

 Chest auscultation reveals fine crackles at the left base, but she is not dyspnoeic.

Rapid strep test is negative, and routine bacterial culture is pending.

 What is the most likely diagnosis?