Likely diagnosis

• Group A beta-hemolytic Streptococcus (GABHS), the main bacterial cause.
• Viral, such as adenovirus.
• EBV infectious mononucleosis.

Step 1: Clinical score (Centor/McIsaac)

• Score 0-1: no test and no antibiotic.
• Score 2 or more: test for GAS.

Step 2: Routine investigations

1. Rapid antigen detection test (RADT) or molecular GAS test on a throat swab.
   • This is the first-line test.
   • In trials, sensitivity is about 60-80% and specificity about 90% (Textbook of Family Medicine 9e, p. 429).
   • A positive result confirms GAS. A negative result is usually accepted in adults, because GAS is less common and rheumatic fever risk is lower.
2. Throat culture on 5% sheep blood agar.
   • This is the gold standard, with about 96% sensitivity, read at 24 hours.
   • Send it if the RADT is negative and suspicion is high, or for diagnostic confirmation.
3. CBC with differential.
   • Lymphocytosis with atypical lymphocytes points to EBV.
   • Neutrophilia is non-specific.
4. CRP. It is an optional marker of bacterial infection.
5. Monospot (heterophile antibody).
   • Send it if there is posterior cervical or generalized lymphadenopathy, marked fatigue, splenomegaly, palatal petechiae, or fever that persists beyond about 5-7 days.
   • If the Monospot is negative but suspicion remains, send EBV VCA IgM.
6. Liver function tests, if mononucleosis is suspected.

Step 3: Add-on tests based on history and findings

• HIV antibody/antigen (4th generation): for risk factors, or if the picture looks like mononucleosis (acute retroviral syndrome mimics it).
• Gonorrhea NAAT or culture from the throat: for oral sexual exposure.
• Syphilis serology: for secondary syphilis, if risk factors or a rash are present.
• Influenza/COVID-19 test: during season or with cough and myalgia.
• Diphtheria culture on special media: for a grey adherent pseudomembrane, unvaccinated status, or travel to an endemic area. Don't remove the membrane (K J Lee's Essential Otolaryngology, p. 684-685).
• Contrast CT neck: only if you suspect peritonsillar abscess or deep neck infection. Look for trismus, a "hot potato" voice, uvular deviation, unilateral swelling, or severe pain with drooling (Textbook of Family Medicine 9e, p. 428).
• Blood culture, lactate, renal function: only if he looks toxic or septic.

Practical points

• The duration in your question reads "e days". I assumed a few days. Fever beyond 5-7 days should raise suspicion of mononucleosis or an abscess.
• Treatment depends on the result. If GAS is confirmed, penicillin or amoxicillin is first line, with a macrolide or cephalosporin as alternatives. Don't give amoxicillin or ampicillin if EBV is possible, because it can cause a rash.
• Check for airway compromise, inability to swallow, and signs of abscess at the bedside.

Diagnosis

• Group A beta-hemolytic Streptococcus (GAS), the main bacterial cause. Onset is typically sudden, with sore throat, painful swallowing, fever, a purulent tonsillar exudate and tender anterior cervical nodes (Textbook of Family Medicine 9e, p. 428).
• Viral, such as adenovirus. This is the most common cause overall. Even with exudates, GAS is documented in under 10% of adults.
• EBV infectious mononucleosis. It is less likely this early, but it also causes exudative tonsillitis.

Investigations

1. Clinical score first (Centor/McIsaac)

• Fever +1, tonsillar exudate +1, tender anterior cervical nodes +1, no cough +1.
• Age 40 adds 0 points.
• He already has at least 2 points. A score of 2 or more means testing for GAS. A score of 0-1 means no test and no antibiotic.
• The 2025 IDSA update supports using a scoring system to decide who gets tested.

2. Routine tests to send

3. Only if indicated

• Monospot or EBV VCA IgM: send if he has posterior cervical or generalized nodes, marked fatigue, splenomegaly or palatal petechiae. At day 3 the heterophile test can be falsely negative, so repeat it in 1-2 weeks if suspicion persists.
• LFTs: if mononucleosis is likely.
• HIV 4th-generation test and syphilis serology: if there are risk factors or a mononucleosis-like picture with rash.
• Throat gonorrhea NAAT: if there is oral sexual exposure.
• Influenza/COVID-19 test: if cough, myalgia or local outbreaks.
• Diphtheria culture: only for a grey adherent pseudomembrane, no vaccination or travel to an endemic area.
• Contrast CT neck: only if you suspect peritonsillar abscess (trismus, "hot potato" voice, uvular deviation, unilateral swelling, drooling).
• Blood cultures, lactate, renal function: only if he looks toxic or septic.

Practical points

• If GAS is confirmed, penicillin or amoxicillin is first line. A macrolide or cephalosporin is the alternative.
• Avoid amoxicillin or ampicillin if EBV is possible, because it can cause a rash.
• Look for red flags: airway compromise, inability to swallow fluids, and signs of abscess. These need urgent assessment.
• If fever persists beyond 5-7 days or he worsens on treatment, reconsider mononucleosis or peritonsillar abscess.