Viral Upper Respiratory Tract Infection (URTI): This diagnosis is the most likely due to the combination of symptoms such as dry cough, sore throat, fever, and headache, which are commonly seen in viral infections like the common cold or influenza.

Influenza: Similar to URTI, influenza presents with fever, cough, sore throat, and fatigue. The presence of systemic symptoms like nausea and diarrhea could also suggest influenza, especially during flu season.
COVID-19: Given the current pandemic, COVID-19 should be considered, especially with symptoms like dry cough, fever, and fatigue. However, the presence of ear pain is less typical.
Strep Throat: Although primarily known for causing a sore throat, strep throat can also present with fever, headache, and fatigue. The absence of a cough might make this less likely, but it's still a consideration, especially if the sore throat is severe.

Pneumonia: While less likely given the absence of productive cough or shortness of breath, pneumonia can present with dry cough, fever, and fatigue. It's crucial to consider this diagnosis due to its potential severity and need for prompt antibiotic treatment.
Meningitis: Although the symptoms don't perfectly align (typically severe headache, fever, and stiff neck), meningitis is a life-threatening condition that must be ruled out, especially if there's any suspicion of neurological involvement.
Sepsis: If the patient shows signs of severe illness, such as high fever, significant fatigue, and nausea/vomiting, sepsis should be considered, as it can arise from any infection and is life-threatening.

Lemierre's Syndrome: A rare condition characterized by a sore throat followed by fever, cough, and neck pain due to a bacterial infection. It's less likely but should be considered if the patient doesn't respond to typical treatments for URTI.
Tuberculosis (TB): Although rare in many parts of the world, TB can present with chronic cough, fever, and fatigue. The presence of ear pain and diarrhea would be unusual, but TB is a consideration in endemic areas or in individuals with risk factors.
Lymphoma: Certain types of lymphoma can present with systemic symptoms like fever, fatigue, and nausea, along with localized symptoms like sore throat or ear pain due to lymph node involvement. This would be a rare and unlikely diagnosis without other supporting evidence.