A new mother expresses her concerns because her 1-day-old newborn has been having feeding difficulties. The child vomits after every feeding and has had a continuous cough since shortly after birth. The mother denies any greenish coloration of the vomit and says that it is only composed of whitish milk that the baby just had. The child exhibits these coughing spells during the exam, at which time the physician notices the child’s skin becoming cyanotic. The mother states that the child was born vaginally with no complications, although her records show that she had polyhydramnios during her last ultrasound before the delivery. Which of the following is the most likely cause of the patient’s symptoms?

Defective formation of the esophagus with tracheoesophageal connection

Failure of recanalization of duodenum

Obstruction due to failure of rotation of pancreatic tissue

Hypertrophy of the pyloric sphincter

Failure of neural crest cells to migrate into the myenteric plexus

A 15-year-old boy is brought to the Emergency department by ambulance from school. He started the day with some body aches and joint pain but then had several episodes of vomiting and started complaining of a terrible headache. The school nurse called for emergency services. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. He is a good student and enjoys sports. At the hospital, his blood pressure is 120/80 mm Hg, heart rate is 105/min, respiratory rate is 21/min, and his temperature is 38.9°C (102.0°F). On physical exam, he appears drowsy with neck stiffness and sensitivity to light. Kernig’s sign is positive. An ophthalmic exam is performed followed by a lumbar puncture. An aliquot of cerebrospinal fluid is sent to microbiology. A gram stain shows gram-negative diplococci. A smear is prepared on blood agar and grows round, smooth, convex colonies with clearly defined edges. Which of the following would identify the described pathogen?

Oxidase-positive and ferments glucose and maltose

Oxidase-positive test and ferments glucose only

Catalase-negative and oxidase-positive

No growth on Thayer-Martin medium

A 2-year-old boy presents for a routine checkup. The patient’s mother says that he has been ‘under the weather’ for the past few days. She did not measure his temperature at home but states that he has felt warm. She denies any episodes of diarrhea or vomiting. No significant past medical history or current medications. The patient attends daycare. He is due for a hepatitis A vaccine. The patient was born at term with no prenatal or perinatal complications. The vital signs include: temperature 37.8°C (100.1°F), blood pressure 112/62 mm Hg, pulse 80/min, respiratory rate 18/min, and oxygen saturation 99% on room air. The patient is alert and responsive. The physical exam is unremarkable. Which of the following is the most appropriate next step in the management of this patient?

Administer the hepatitis A vaccine

Strep rapid antigen detection test

Delay the hepatitis A immunization until next visit

A 26-year-old woman at 30 weeks 2 days of gestational age is brought into the emergency room following a seizure episode. Her medical records demonstrate poorly controlled gestational hypertension. Following administration of magnesium, she is taken to the operating room for emergency cesarean section. Her newborn daughter’s APGAR scores are 7 and 9 at 1 and 5 minutes, respectively. The newborn is subsequently taken to the NICU for further management and monitoring. Ten days following birth, the baby begins to refuse formula feedings and starts having several episodes of bloody diarrhea despite normal stool patterns previously. Her temperature is 102.2°F (39°C), blood pressure is 84/53 mmHg, pulse is 210/min, respirations are 53/min, and oxygen saturation is 96% on room air. A physical examination demonstrates a baby in mild respiratory distress and moderate abdominal distention. What do you expect to find in this patient?

Gas within the walls of the small or large intestine on radiograph

Positive blood cultures of group B streptococcus

High levels of cow's milk-specific IgE

Absence of ganglion cells on rectal biopsy

Double bubble sign on abdominal radiograph

Four days after delivery, a 1400-g (3-lb 1-oz) newborn has a tonic seizure that lasts for 30 seconds. Over the past 24 hours, he has become increasingly lethargic. He was born at 31 weeks' gestation. Antenatal period was complicated by chorioamnionitis. Apgar scores were 4 and 5 at 1 and 5 minutes, respectively. He appears ill. His pulse is 130/min, respirations are 53/min and irregular, and blood pressure is 67/35 mm Hg. Examination shows a bulging anterior fontanelle. The pupils are equal and react sluggishly to light. Examination shows slow, conjugate back and forth movements of the eyes. Muscle tone is decreased in all extremities. The lungs are clear to auscultation. Which of the following is the most likely underlying cause?

Galactose-1-phosphate uridylyltransferase deficiency

Phenylalanine hydroxylase deficiency

Apoptosis of lower motor neurons

A 31-year-old man and his wife were referred to a genetic counselor. They are concerned about the chance that their children are likely to inherit certain conditions that run in their families. The wife's father and grandfather are both healthy, but her grandfather cannot see the color red. The husband is unaware if any member of his family has the same condition. The geneticist provides some details about genetic diseases and inheritance patterns, then orders lab tests to analyze the gene mutations carried by both partners. Which of the following are the correct terms regarding the genotype and phenotype of males affected by the condition described?

Homozygote; reduced or incomplete penetrance

Heterozygotes; reduced or incomplete penetrance

Hemizygous; reduced or incomplete penetrance

Red flags in pediatric presentations for USMLE Step 3: High-Yield Diagnosis Lessons

General Red Flags - The Big Worries

Fever of Unknown Origin: Especially in neonates (<28 days) or immunocompromised.
Altered Mental Status: Lethargy, irritability, inconsolability, or post-ictal state.
Respiratory Distress: Nasal flaring, grunting, retractions, tachypnea, or cyanosis.
Circulatory Compromise: Tachycardia, hypotension, delayed capillary refill (>2 seconds), weak pulses.
Dehydration: Sunken fontanelle, dry mucous membranes, ↓ skin turgor, no tears.
Non-blanching Rash: Petechiae or purpura; could signal meningococcemia.
Suspected Non-Accidental Trauma (NAT): Inconsistent history, specific fracture patterns (e.g., spiral), or multiple bruises.

⭐ Any neonate (<28 days old) with a fever >38°C (100.4°F) is considered to have sepsis until proven otherwise and requires a full septic workup.

Petechial rash on child with meningococcemia

Age-Specific Alerts - Cradle to Crayons

Neonate (< 28 days)
- Fever > 38°C (100.4°F) or hypothermia < 36.5°C (97.7°F)
- Lethargy, poor feeding, or decreased tone
- Bulging or tense fontanelle
- Persistent jaundice (> 2 weeks)
- Bilious (green) vomiting
- No wet diapers for > 24 hours
Infant (1-12 months)
- Inconsolable crying, especially with drawing up of legs (intussusception)
- New-onset seizures
- Respiratory distress: nasal flaring, grunting, retractions
- Developmental regression (loss of milestones)
- Projectile, non-bilious vomiting (pyloric stenosis)
Toddler & Preschooler (1-5 years)
- Limp or refusal to bear weight
- Unexplained petechiae or bruising
- Severe headache, especially with morning vomiting or ataxia
- Unilateral, painful proptosis (orbital cellulitis)

⭐ Exam Favorite: Any fever > 38°C (100.4°F) in a neonate (< 28 days old) is considered sepsis until proven otherwise. This requires an immediate, full septic workup (blood, urine, and CSF cultures) and admission for empiric IV antibiotics.

System-Specific Warnings - Head to Toe Woes

Head & Neck
- Bulging fontanelle, rapid head growth, sunsetting eyes: ↑ Intracranial Pressure (ICP).
- Tripod position, drooling, muffled voice: Epiglottitis.
- Nuchal rigidity, photophobia: Meningitis.
Eyes & Ears
- Leukocoria (white pupillary reflex): Retinoblastoma.
- Postauricular swelling, tenderness: Mastoiditis.
Cardiopulmonary
- New murmur + fever: Infective endocarditis.
- Weak femoral pulses, BP discrepancy (arm > leg): Coarctation of Aorta.
- Silent chest (no wheezing during asthma attack): Impending respiratory failure.
Gastrointestinal
- Bilious (green) vomiting: Bowel obstruction (e.g., volvulus).
- Currant jelly stool, sausage-shaped mass: Intussusception.
- Painless rectal bleeding: Meckel's diverticulum.
Genitourinary & Musculoskeletal
- Acute scrotal pain & swelling: Testicular torsion.
- Palpable, non-tender abdominal mass: Wilms tumor.
- Limp or refusal to bear weight: Septic arthritis, fracture, malignancy.
Dermatology & Heme
- Non-blanching rash (petechiae/purpura): Meningococcemia, HSP.

⭐ Bilious vomiting in a neonate is a surgical emergency until proven otherwise, most concerning for malrotation with midgut volvulus.

Epiglottitis: Clinical Presentation, Anatomy, and Radiograph

Fever in a neonate (<28 days) is a sepsis equivalent requiring an immediate, full workup.

Bilious vomiting signals a potential surgical emergency like malrotation with volvulus or duodenal atresia.

A non-blanching rash (petechiae/purpura) is a major warning for meningococcemia or other sepsis.

Lethargy, inconsolability, or a weak cry are critical indicators of serious systemic illness or CNS infection.

Sudden limp or refusal to bear weight warrants urgent investigation for septic arthritis or occult fracture.

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