Yellow mucus phlegm in the throat may be caused by acute pharyngolaryngitis, acute tonsillitis, sinusitis, bronchitis, pneumonia and other causes. 1. Acute pharyngolaryngitis: the throat is infected by pathogenic bacteria triggering acute inflammation, resulting in local congestion, edema, increased secretions, so there is always yellow mucus in the throat, sore throat and other symptoms. 2. Acute tonsillitis: acute inflammation of the tonsils with purulent secretions, so there is always yellow mucus in the throat, accompanied by sore throat, fever and other symptoms. 3. Sinusitis: inflammation of the sinuses, nasal secretions increase to flow into the throat causing yellow mucus in the throat, accompanied by nasal congestion, runny nose, headache and other symptoms. 4. Bronchitis: Inflammation of the bronchial tubes causes coughing and phlegm, so there is always yellow mucus in the throat. 5. Pneumonia: Acute inflammation of the lungs caused by bacterial and viral infections leads to coughing and phlegm, so there is always yellow mucus in the throat, accompanied by shortness of breath, dyspnea and other symptoms. There may be other reasons for the yellow mucus in the throat, so it is recommended that you seek medical advice to identify the cause of the problem and follow the doctor’s instructions.