Give your differential diagnosis.

Hypothetical Clinical Scenario

A 25-year-old male presents to the emergency department with a 3-day history of high fever (102°F), severe headache, myalgia (muscle pain), and a maculopapular rash that started on the trunk and is now spreading to the limbs. He reports no recent travel history but mentions a recent mosquito bite. He denies any chronic medical conditions and is not on any regular medications.

Differential Diagnosis

Based on the presented symptoms, the following differential diagnoses should be considered:

1. Dengue Fever

Rationale: High fever, severe headache, myalgia, and rash are classic symptoms of dengue fever. The recent mosquito bite further supports this possibility. Dengue is endemic in many tropical and subtropical regions.

2. Chikungunya Fever

Rationale: Similar to dengue, chikungunya presents with fever, headache, myalgia, and rash. However, chikungunya is often characterized by more severe and prolonged joint pain (arthralgia).

3. Zika Virus Infection

Rationale: Zika virus can cause similar symptoms to dengue and chikungunya, although it is often milder. The rash is typically maculopapular and can be itchy.

4. Measles

Rationale: Although less common due to vaccination, measles can present with fever, rash, and headache. Measles rash typically starts on the face and spreads downwards. Koplik spots (small white spots inside the mouth) are a characteristic finding.

5. Typhoid Fever

Rationale: Typhoid fever can cause high fever, headache, and rash (rose spots). It is typically associated with contaminated food or water.

6. Leptospirosis

Rationale: Leptospirosis is a bacterial infection transmitted through contact with contaminated water or animal urine. Symptoms include fever, headache, myalgia, and rash. It can lead to severe complications like kidney failure and jaundice.

7. Early Stage Viral Exanthems (e.g., Rubella, Fifth Disease)

Rationale: Various viral infections can cause fever and rash. These are generally milder but need to be considered.

Diagnostic Investigations

To narrow down the differential diagnosis, the following investigations are crucial:

• Complete Blood Count (CBC): Dengue often causes thrombocytopenia (low platelet count) and leukopenia (low white blood cell count).
• Dengue NS1 Antigen Test & IgM/IgG Antibodies: To confirm or rule out dengue fever.
• Chikungunya PCR & IgM/IgG Antibodies: To confirm or rule out chikungunya fever.
• Zika Virus PCR & IgM/IgG Antibodies: To confirm or rule out Zika virus infection.
• Measles IgM/IgG Antibodies: To confirm or rule out measles.
• Widal Test: To assess for typhoid fever (though its sensitivity and specificity are limited).
• Leptospira Serology: To assess for leptospirosis.
• Blood Culture: To rule out bacterial sepsis.
• Liver Function Tests (LFTs): To assess liver involvement, which can occur in several of these infections.
• Renal Function Tests (RFTs): To assess kidney function, particularly important in leptospirosis.

Prioritization of Diagnoses

Initially, Dengue, Chikungunya, and Zika virus should be prioritized due to the mosquito bite history and the typical presentation. Measles should be considered if vaccination history is unknown. Typhoid and Leptospirosis are less likely given the lack of specific risk factors but should remain on the list until ruled out.

Diagnosis	Key Symptoms	Investigations	Probability (Initial)
Dengue Fever	Fever, Headache, Myalgia, Rash, Thrombocytopenia	NS1 Antigen, IgM/IgG	High
Chikungunya Fever	Fever, Headache, Myalgia, Arthralgia, Rash	PCR, IgM/IgG	Medium
Zika Virus Infection	Fever, Headache, Myalgia, Rash	PCR, IgM/IgG	Medium
Measles	Fever, Rash, Koplik Spots	IgM/IgG	Low (if vaccinated)