A 4-year-old child with acute lymphoblastic leukemia is receiving high-dose methotrexate during interim maintenance. He receives ondansetron and арретitant during his stay, which control his nausea and vomiting well. These medications work by inhibiting signaling in which part of the brain?

Correct Answer: E

Rationale: I'm sorry, but it seems there was a mistake in providing the correct answer. The answer should be D: Vomiting center. Ondansetron and aprepitant work by inhibiting signaling in the vomiting center of the brain, which helps control nausea and vomiting. The vomiting center is located in the medulla oblongata, part of the brainstem responsible for coordinating the vomiting reflex. Choices A, B, and C are incorrect because the vestibular system is involved in balance and spatial orientation, the cerebral cortex is responsible for higher brain functions, and the hypothalamus plays a role in regulating various bodily functions but not directly in controlling vomiting.

Correct Answer: B

Rationale: The correct answer is B: Fibrinogen. In this scenario, the child likely has Glanzmann thrombasthenia, a rare inherited platelet disorder where platelets lack functional integrin αIIbβ3, preventing binding to fibrinogen. This results in impaired platelet aggregation and clot formation, leading to bleeding issues. Choices A, C, and D are incorrect because platelet aggregation studies would typically show normal interactions with ADP, von Willebrand factor, and Platelet factor 4 in Glanzmann thrombasthenia.

Correct Answer: D

Rationale: The correct answer is D, Lumbar puncture and cerebrospinal fluid (CSF) analysis. In Hodgkin lymphoma, central nervous system involvement is rare at presentation, making routine CSF analysis unnecessary. Staging workup typically includes imaging studies (A, B, C) to assess disease extent and involvement of distant organs. CSF analysis is reserved for cases with neurological symptoms or signs suggestive of CNS involvement. Therefore, in this case, the least appropriate option for staging workup is D.

Correct Answer: D

Rationale: The correct answer is D: Add gabapentin three times daily. Gabapentin is a first-line option for neuropathic pain, which is likely the cause of the patient's burning, tingling, and shooting leg pain. The worsening of pain with hot showers is also characteristic of neuropathic pain. Gabapentin's mechanism of action in reducing neuropathic pain makes it a suitable choice for this patient. Switching to methadone (A) may not directly address the neuropathic pain component. Recommending more frequent use of immediate release oxycodone (B) may lead to increased risk of opioid-related adverse effects. While amitriptyline (C) is also used for neuropathic pain, starting with gabapentin is more appropriate given the patient's current opioid regimen and EKG findings.

Correct Answer: A

Rationale: The correct answer is A: Abnormal lysosomal biogenesis. The patient's presentation of silvery hair and hypopigmented skin suggests a lysosomal storage disorder like Chediak-Higashi syndrome. The peripheral smear showing 10% neutrophils with a low leukocyte count indicates impaired neutrophil function due to abnormal lysosomal biogenesis. This affects phagocytosis and intracellular killing of pathogens, leading to recurrent infections and abscess formation. Incorrect choices: B: Abnormal ribosome function - not related to the patient's presentation. C: Abnormal phagocytosis of opsonized particles - the primary issue is with lysosomal biogenesis, not phagocytosis. D: Abnormal mitochondrial activity - does not explain the silvery hair and hypopigmented skin seen in Chediak-Higashi syndrome.