A 22-year-old female with no past medical history presents to her primary care physician with a 3-day history of knee pain. She denies any recent injury or trauma. On physical examination her knee is warm, erythematous, and has diminished range of movement. The patient reports having multiple sexual partners over the last year and does not use protection regularly. Her blood pressure is 124/85 mmHg, heart rate is 76/min, and temperature is 38.3℃ (101.0℉). A joint aspiration is performed and a growth of gram-negative diplococci is noted on bacterial culture. What is the treatment of choice for this patient's condition?

Ceftriaxone monotherapy and joint aspiration

Nafcillin monotherapy and joint aspiration

A 42-year-old woman comes to the emergency department because of worsening severe pain, swelling, and stiffness in her right knee for the past 2 days. She recently started running 2 miles, 3 times a week in an attempt to lose weight. She has type 2 diabetes mellitus and osteoporosis. Her mother has rheumatoid arthritis. She drinks one to two glasses of wine daily. She is sexually active with multiple partners and uses condoms inconsistently. Current medications include metformin and alendronate. She is 161 cm (5 ft 3 in) tall and weighs 74 kg (163 lb); BMI is 29 kg/m2. Her temperature is 38.3°C (100.9°F), pulse is 74/min, and blood pressure is 115/76 mm Hg. She appears to be in discomfort and has trouble putting weight on the affected knee. Physical examination shows a 2-cm, painless ulcer on the plantar surface of the right toe. The right knee is swollen and tender to palpation. Arthrocentesis of the right knee with synovial fluid analysis shows a cell count of 55,000 WBC/μL with 77% polymorphonuclear (PMN) cells. Which of the following is the most likely underlying cause of this patient's presenting condition?

Hematogenous spread of infection

Autoimmune response to bacterial infection

Intra-articular deposition of urate crystals

Direct inoculation of infectious agent

A 32-year-old man comes into your office because of pain in his right knee, left elbow, and left wrist. It started about a week ago but has particularly localized to his wrist. The patient states that he has 2 sexual partners. He states he has also had some white discharge from his penis with pruritis and pain during urination. His temperature is 97.6°F (36.4°C), blood pressure is 124/84 mmHg, pulse is 80/min, respirations are 12/min, and oxygen saturation is 98% on room air. Physical exam reveals pain upon palpation of the patient's left wrist which also appears erythematous and swollen. What is the best next step in management for this patient?

Azithromycin, ceftriaxone, and vancomycin

An 11-year-old boy presents with fever and joint pain for the last 3 days. His mother says that he had a sore throat 3 weeks ago but did not seek medical care at that time. The family immigrated from the Middle East 3 years ago. The patient has no past medical history. The current illness started with a fever and a swollen right knee that was very painful. The following day, his knee improved but his left elbow became swollen and painful. While in the waiting room, his left knee is also becoming swollen and painful. Vital signs include: temperature 38.7°C (101.6°F), and blood pressure 110/80 mm Hg. On physical examination, the affected joints are swollen and very tender to touch, and there are circular areas of redness on his back and left forearm (as shown in the image). Which of the following is needed to establish a diagnosis of acute rheumatic fever in this patient?

Positive anti-streptococcal serology

Elevated erythrocyte sedimentation rate (ESR)

No other criterion is needed to establish the diagnosis of acute rheumatic fever

An 11-year-old boy presents to the emergency department with sudden pain. The patient's parents state the child suddenly felt pain in his hands an hour ago. He has not eaten anything new lately. He did play football earlier this morning and admits to being tackled forcefully multiple times. The child is doing well in school and is proud that he has a new girlfriend. The child has a past medical history of obesity and is not currently on any medications. His temperature is 100°F (37.8°C), blood pressure is 120/68 mmHg, pulse is 100/min, respirations are 11/min, and oxygen saturation is 98% on room air. Cardiopulmonary exam is within normal limits. On physical exam of the patient's extremities, there was painful and symmetrical swelling of his hands and feet but no rashes were present. The patient is started on analgesics. Which of the following is the most likely cause of this patient's presentation?

Altered red blood cell morphology

Benign edema secondary to trauma

An 8-year-old boy presents to his pediatrician accompanied by his father with a complaint of chronic cough. For the past 2 months he has been coughing up yellow, foul-smelling sputum. He has been treated at a local urgent care center for multiple episodes of otitis media, sinusitis, and bronchitis since 2 years of age. His family history is unremarkable. At the pediatrician's office, his temperature is 99.2°F (37.3°C), blood pressure is 110/84 mmHg, pulse is 95/min, and respirations are 20/min. Inspection shows a young boy who coughs occasionally during examination. Pulmonary exam demonstrates diffuse wheezing and crackles bilaterally. Mild clubbing is present on the fingers. The father has brought an electrocardiogram (ECG) from the patient’s last urgent care visit that shows pronounced right axis deviation. Which of the following is the most likely etiology of this patient’s condition?

Failure of neural crest cell migration

Maldevelopment of pharyngeal pouches

Defective maturation of B-lymphocytes

A 3-year-old girl is brought to the emergency room because of a 5-day history of high fever and fatigue. During this time she has been crying more than usual and eating less. Her mother says that the child has also complained about pain in her arms and legs for the past 3 days. She was born at term and has been otherwise healthy. She appears ill. Her temperature is 39.5°C (103.1°F), pulse is 128/min, and blood pressure is 96/52 mm Hg. The lungs are clear to auscultation. A grade 3/6 systolic murmur is heard at the apex. There is mild tenderness to palpation of the left upper quadrant with no guarding or rebound. The spleen is palpated 3 cm below the left costal margin. There is no redness or swelling of the joints. Laboratory studies show: Hemoglobin 11.8 g/dL Leukocyte count 16,300/mm3 Platelet count 220,000/mm3 Erythrocyte sedimentation rate 50 mm/h Serum Glucose 96 mg/dL Creatinine 1.7 mg/dL Total bilirubin 0.4 mg/dL AST 18 U/L ALT 20 U/L Urine Protein 2+ RBC casts rare RBC 10/hpf WBC 1–2/hpf Which of the following is the most appropriate next step in management?

Obtain 3 sets of blood cultures

Obtain a transesophageal echocardiography

Administer intravenous vancomycin

Bone and joint infections for USMLE Step 2 CK: High-Yield Pediatrics Lessons

Osteomyelitis - Bone Under Siege

Vascular supply of proximal humerus in children

Pathogenesis: Primarily hematogenous spread to the metaphysis of long bones (e.g., femur, tibia) due to rich, slow-flowing sinusoidal veins.
Etiology:
- Most common: Staphylococcus aureus.
- Neonates: Group B Streptococcus, E. coli.
- Sickle Cell Disease: Salmonella spp., S. aureus.
Diagnosis:
- Labs: ↑ ESR & CRP are early indicators.
- Imaging: X-ray findings (periosteal reaction, lytic lesions) appear late (after 10-14 days). MRI is the investigation of choice for early diagnosis.
- Gold Standard: Bone biopsy and culture.

⭐ In neonates, infection often spreads to the epiphysis and joint space, causing concomitant septic arthritis, due to transphyseal blood vessels.

Management: Prolonged IV antibiotics (typically 4-6 weeks). Surgical drainage is required for abscesses or sequestra (necrotic bone).

Septic Arthritis - Joint Jeopardy

Etiology: S. aureus is the most common cause overall.
- Neonates: GBS, S. aureus, Gram-negatives.
- Children <5 yrs: S. aureus, S. pyogenes.
- Adolescents: Consider N. gonorrhoeae.
Clinical: Acute fever, severe monoarticular pain (hip/knee), pseudoparalysis, and refusal to bear weight.
Diagnosis (Kocher's Criteria for Hip):
- Fever > 38.5°C
- Inability to bear weight
- ESR > 40 mm/hr
- WBC > 12,000/mm³
- ≥3 criteria → 93% probability.

⭐ Gold Standard: Joint aspiration showing purulent fluid (WBC > 50,000/mm³, >75% neutrophils), low glucose, and positive culture.

Treatment: Urgent surgical drainage (arthrotomy/arthroscopy) + IV antibiotics.

Diagnosis - Infection Detectives

Lab Markers:
- ↑ ESR > 40 mm/hr
- ↑ CRP > 20 mg/L
- ↑ TLC (Leukocytosis)
- Blood Culture: Positive in only ~50% cases.
Imaging:
- X-ray: May be normal for 7-10 days. Late signs: lytic lesions, periosteal reaction.
- Ultrasound: Detects joint effusion.
- MRI: Most sensitive for early diagnosis of osteomyelitis.
Gold Standard: Aspiration of pus from bone/joint for culture.

X-ray of pediatric wrist with osteomyelitis

💡 Kocher Criteria for Septic Arthritis

⭐ With all 4 Kocher criteria positive, the probability of septic arthritis is 99%. With 3 positive, it's 93%.

Management - Antibiotic Arsenal

Empirical Therapy: Guided by age and likely pathogens.

Duration:
- Acute Osteomyelitis: 4-6 weeks (minimum 2 weeks IV).
- Septic Arthritis: 2-3 weeks.
Special Scenarios:
- Sickle Cell Disease: Add 3rd Gen Cephalosporin (e.g., Ceftriaxone) to cover Salmonella.
- Puncture Wound (foot): Cover Pseudomonas.

⭐ In children < 4 years, Kingella kingae is a common cause of septic arthritis. It is often gram-stain negative and requires nucleic acid amplification tests (NAAT) for diagnosis.

S. aureus is the most common cause of acute osteomyelitis and septic arthritis in children.

In neonates, consider Group B Strep and E. coli.

Salmonella is the classic cause of osteomyelitis in sickle cell disease.

Kingella kingae is a key pathogen for septic arthritis in children <4 years old.

MRI is the most sensitive imaging modality for detecting early osteomyelitis.

Pott's spine (TB) most commonly affects the thoracic spine.

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