A father brings in his 7-year-old twin sons because they have a diffuse rash. They have several papules, vesicles, pustules, and crusts on their scalps, torso, and limbs. The skin lesions are pruritic. Other than that, the boys appear to be well. The father reports that several children in school have a similar rash. The family recently returned from a beach vacation but have not traveled internationally. Both boys have stable vital signs within normal limits. What is the most common complication of the infection the boys appear to have?

Bacterial superinfection of skin lesions

A 65-year-old man presents to his primary care physician for a rash. He states that for the past several days he has felt burning and itching around his eye. Yesterday, he noticed that a rash had formed. Review of systems is notable for mild diarrhea for the past week. The patient has a past medical history of diabetes, asthma, seasonal allergies, and hypertension. He is not currently taking any medications. Physical exam is notable for a vesicular rash surrounding the orbit. Which of the following is the best next step in management?

Removal of gluten containing products from the diet

A 67-year-old man presents to the emergency department with confusion. The patient is generally healthy, but his wife noticed him becoming progressively more confused as the day went on. The patient is not currently taking any medications and has no recent falls or trauma. His temperature is 102°F (38.9°C), blood pressure is 126/64 mmHg, pulse is 120/min, respirations are 17/min, and oxygen saturation is 98% on room air. Physical exam is notable for a confused man who cannot participate in a neurological exam secondary to his confusion. No symptoms are elicited with flexion of the neck and jolt accentuation of headache is negative. Initial laboratory values are unremarkable and the patient's chest radiograph and urinalysis are within normal limits. An initial CT scan of the head is unremarkable. Which of the following is the best next step in management?

CT angiogram of the head and neck

Vancomycin, ceftriaxone, ampicillin, and dexamethasone

A 17-year-old male comes to the physician because of painful genital sores, malaise, and fever for 3 days. He is sexually active with 3 female partners and does not use condoms consistently. His temperature is 38.3°C (101°F). Physical examination shows tender lymphadenopathy in the left inguinal region and multiple, punched-out ulcers over the penile shaft and glans. Microscopic examination of a smear from the ulcer is most likely to show which of the following?

Eosinophilic intranuclear inclusions

Bipolar-staining intracytoplasmic inclusions

Eosinophilic intracytoplasmic inclusions

Basophilic intranuclear inclusions

Basophilic intracytoplasmic inclusions

A 16-year-old male is brought to the clinic by his mother for the complaints of fever, nonproductive cough, fatigue, lack of appetite, and sore throat for the past 2 months. Several other students at his high school have had similar symptoms. Physical exam shows a whitish membrane in his oropharynx, bilateral enlarged cervical lymphadenopathy, and mild splenomegaly. Which of the following tests is most likely to diagnose his condition?

Enzyme-linked immunosorbent assay

Herpesviruses (HSV, VZV, CMV, EBV, HHV-6/7/8) for USMLE Step 3: High-Yield Microbiology Lessons

Herpesviruses - The Latent Menace

Core Features: All are enveloped, icosahedral, dsDNA viruses that establish lifelong latent infections.
HSV-1/2: Oral/genital ulcers; devastating temporal lobe encephalitis. Latency in sensory ganglia.
VZV (HHV-3): Varicella (chickenpox) → Zoster (shingles) reactivation in a dermatome.
EBV (HHV-4): Infectious mononucleosis; associated with Burkitt lymphoma & nasopharyngeal carcinoma.
CMV (HHV-5): Retinitis/colitis in immunocompromised; "owl's eye" inclusions.
HHV-6/7: Roseola infantum (high fever followed by rash).
HHV-8: Kaposi’s sarcoma.

Cowdry Type A Inclusion Bodies

⭐ CMV is the most common congenital viral infection in the U.S., a major cause of sensorineural hearing loss and neurodevelopmental delay.

Alpha Group (HSV, VZV) - Skin & Nerve Invaders

Hallmarks: Fast-growing, cytolytic, and establish lifelong latency in sensory nerve ganglia (trigeminal for HSV-1, sacral for HSV-2, dorsal root for VZV).
HSV-1 & HSV-2:
- Clinical:
  - HSV-1: Gingivostomatitis, keratoconjunctivitis, esophagitis.
  - HSV-2: Genital & neonatal herpes.
- Diagnosis: PCR is gold standard. Tzanck smear shows multinucleated giant cells.
Varicella-Zoster (VZV):
- Primary (Varicella): "Chickenpox" with asynchronous ("dewdrop on a rose petal") vesicular rash.
- Reactivation (Zoster): "Shingles" in a painful, unilateral dermatomal distribution.

Tzanck smear: multinucleated giant cells in HSV infection

⭐ HSV-1 encephalitis is the most common cause of fatal sporadic encephalitis, classically causing hemorrhagic necrosis of the temporal lobes.

Beta Group (CMV, HHV-6/7) - The Slow & Stealthy

Cytomegalovirus (CMV/HHV-5)
- Transmission: Saliva, urine, blood, sexual, vertical.
- Syndromes:
  - Immunocompetent: Mononucleosis-like (heterophile Ab-negative).
  - Immunocompromised: Retinitis ("pizza-pie"), pneumonitis, esophagitis (linear ulcers).
  - Congenital: Blueberry muffin rash, hepatosplenomegaly, sensorineural deafness.
- Tx: Ganciclovir, valganciclovir.
HHV-6 / HHV-7
- Disease: Roseola Infantum (Exanthem Subitum).
- Presentation: 3-5 days high fever → fever breaks → rash appears (trunk first).
- Common cause of febrile seizures.

CMV owl's eye inclusion bodies in histology

⭐ Histology Classic: CMV is famous for its large cells with prominent basophilic intranuclear "Owl's eye" inclusion bodies.

Gamma Group (EBV, HHV-8) - Oncogenic Opportunists

Epstein-Barr Virus (EBV / HHV-4)
- Causes Infectious Mononucleosis (fever, pharyngitis, lymphadenopathy); transmitted via saliva.
- Infects B-cells via CD21 receptor.
- Diagnosis: Monospot test (heterophile antibodies), atypical lymphocytes (Downey cells).
Human Herpesvirus 8 (HHV-8)
- Causes Kaposi's Sarcoma, a vascular tumor common in AIDS patients.
- Presents as violaceous lesions on the nose, extremities, and mucous membranes.
- Pathogenesis involves viral-encoded homologues that promote angiogenesis.

⭐ EBV is strongly associated with Burkitt Lymphoma (especially African type, t(8;14) translocation) and nasopharyngeal carcinoma.

Atypical Lymphocytes (Downey Cells) in Mononucleosis

Diagnosis & Antivirals - Lab & Pharmacy

Lab Diagnosis
- PCR: Gold standard for HSV, VZV, CMV detection.
- Tzanck Smear: Multinucleated giant cells (HSV/VZV).
- Biopsy: "Owl's eye" inclusions (CMV), atypical lymphocytes (EBV).
Antiviral Therapy
- Acyclovir/Valacyclovir for HSV/VZV.
- Ganciclovir/Valganciclovir for CMV.
- Foscarnet for ganciclovir-resistant CMV.

⭐ Foscarnet directly inhibits DNA polymerase without needing viral kinase activation, making it effective against resistant HSV/CMV strains.

HSV is the most common cause of sporadic temporal lobe encephalitis.

VZV reactivation causes a painful dermatomal rash (shingles).

CMV in the immunocompromised shows owl's eye inclusions.

EBV is linked to infectious mononucleosis, Burkitt lymphoma, and nasopharyngeal carcinoma.

HHV-6 causes roseola infantum: high fever followed by a rash.

HHV-8 is the causative agent of Kaposi's sarcoma.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play