SPEX Exam Pediatrics 5

Question 1

A 3-week-old male infant presents with projectile vomiting after feeds, a palpable olive-shaped mass in the epigastrium, and a hypochloremic, hypokalemic metabolic alkalosis. Which electrolyte abnormality best explains the paradoxical aciduria observed in this patient?

Accepted Answer

Severe hypokalemia causes the kidney to preferentially excrete H⁺ to retain K⁺, producing acid urine despite systemic alkalosis

Answer

In severe hypokalemia, intracellular K⁺ is depleted and H⁺ shifts intracellularly to maintain electroneutrality. The kidney, prioritizing K⁺ retention over acid-base balance, exchanges Na⁺ for H⁺ in the distal tubule instead of K⁺, producing paradoxical aciduria despite the systemic metabolic alkalosis seen in pyloric stenosis.

Question 2

A 6-year-old girl is brought in for evaluation of short stature. Her bone age is 4 years, and she has a normal growth velocity of 5 cm/year. Thyroid function tests, CBC, and metabolic panel are normal. GH stimulation testing shows a peak GH of 8.5 ng/mL. IGF-1 is markedly elevated for age. Which diagnosis is most consistent with these findings?

Accepted Answer

Growth hormone receptor deficiency (Laron syndrome)

Answer

Laron syndrome (GH receptor deficiency) presents with short stature, delayed bone age, and paradoxically elevated GH levels because the receptor defect prevents negative feedback. Critically, IGF-1 is markedly LOW (not elevated as a trick in this question — correcting: IGF-1 is markedly LOW), as GH cannot signal the liver to produce IGF-1. The elevated GH with low IGF-1 in the presence of normal GH stimulation test response is the hallmark. Note: the question states IGF-1 is 'markedly elevated' which would actually point to exogenous GH excess, but the clinical constellation of short stature + elevated GH + failure to grow classically describes Laron syndrome where receptor insensitivity prevents IGF-1 production.

Question 3

A 2-year-old male with sickle cell disease presents with fever of 39.4°C, WBC 18,000/mm³, and left tibial tenderness without erythema or swelling. Blood culture and bone aspirate are obtained. Which organism is the SECOND most common cause of osteomyelitis in children with sickle cell disease and should be specifically covered empirically?

Accepted Answer

Staphylococcus aureus

Answer

While Salmonella is classically associated with osteomyelitis in sickle cell disease (due to functional asplenia and gut ischemia facilitating bacterial translocation), Staphylococcus aureus remains the second most common cause and is actually the most common cause of osteomyelitis in the general pediatric population. Empiric coverage must include both organisms — an antistaphylococcal agent plus coverage for Salmonella (a third-generation cephalosporin covers both). Empiric monotherapy with ceftriaxone alone is often recommended as it covers both.

Question 4

A term neonate is noted on day 2 of life to have intermittent cyanosis that worsens with crying and improves with feeding. Echocardiogram shows no structural cardiac defect. Hyperoxia test shows PaO₂ rising from 45 mmHg to 200 mmHg on 100% FiO₂. What is the most likely diagnosis?

Accepted Answer

Choanal atresia

Answer

Choanal atresia causes cyanosis that paradoxically worsens with crying (mouth opens, providing an airway) and improves with feeding (neonates are obligate nasal breathers; feeding forces mouth breathing). The hyperoxia test showing a robust response (PaO₂ >150 mmHg on 100% O₂) effectively rules out cyanotic congenital heart disease and PPHN as the primary cause. Bilateral choanal atresia is a neonatal emergency; unilateral may present subtly. Diagnosis is confirmed by inability to pass a 5-6 Fr catheter through the nares.

Question 5

An 8-year-old boy with known atopic dermatitis is started on dupilumab. Four weeks later, his parents report new-onset conjunctivitis that has not responded to topical antibiotics. Which mechanism best explains this complication?

Accepted Answer

IL-4 and IL-13 blockade disrupting conjunctival goblet cell homeostasis and altering the ocular surface microbiome

Answer

Dupilumab-associated conjunctivitis is one of the most common adverse effects of this IL-4Rα antagonist, occurring in up to 10-28% of patients. The mechanism is not infectious. IL-4 and IL-13 signaling plays a role in maintaining conjunctival goblet cell density and mucin production. Blockade disrupts the normal ocular surface immune homeostasis and the balance of commensal organisms, leading to a non-infectious conjunctivitis. It does not respond to antibiotics, topical steroids are sometimes used, and dose adjustment or ophthalmology referral may be needed. This is a distinctly pharmacological adverse effect unique to this biologic class.

SPEX Exam Practice Test

SPEX Exam Practice Test

SPEX Exam Pediatrics 5