A 5-year-old boy is brought to the pediatric clinic by his mother for evaluation of multiple brown patches on his skin. On examination, the child has 7 well-demarcated, uniformly tan macules distributed over his trunk and limbs, each measuring 8–12 mm in greatest diameter. The structure marked **A** in the diagram represents these lesions. Which of the following is the MOST important next step in establishing a diagnosis of Neurofibromatosis Type 1 (NF1) in this child?

Explanation

## Why Examine the axillae and inguinal folds for freckling (Crowe sign) is right The NIH diagnostic criteria (revised 2021) for NF1 require ≥2 of 8 major features for diagnosis. The structure marked **A** — ≥6 café-au-lait macules >5 mm prepubertal or >15 mm postpubertal — represents ONE criterion. This 5-year-old already satisfies criterion 1 (he has 7 macules, each 8–12 mm, which exceeds the 5 mm threshold for prepubertal children). To confirm NF1 diagnosis, the next clinical step is to identify a SECOND criterion. Axillary and inguinal freckling (Crowe sign, criterion 2) is the most readily assessable on physical examination and typically develops by age 3–5 years. Finding this would establish the diagnosis without need for imaging or genetic testing at this stage. (Harrison 21e Ch 471; Nelson 21e) ## Why each distractor is wrong - **Perform slit-lamp examination to look for Lisch nodules and iris hamartomas**: While Lisch nodules (criterion 5) are present in ~95% of adults with NF1, they are often absent in young children and develop later in childhood. This is not the most practical next step in a 5-year-old. - **Obtain MRI of the brain to screen for optic pathway glioma**: MRI is part of surveillance (recommended at diagnosis and as indicated), but it is not the NEXT diagnostic step. Optic pathway gliomas (criterion 4) are present in only 15–20% of NF1 children and are usually asymptomatic. Clinical examination for a second criterion is more efficient. - **Perform genetic sequencing of the NF1 gene on chromosome 17q11**: Genetic confirmation (criterion 8, added in 2021 revision) is a valid diagnostic tool, but it is not the NEXT clinical step. The diagnosis can be established by meeting ≥2 clinical criteria, which is faster and more cost-effective in this setting. **High-Yield:** NF1 diagnosis is clinical (≥2 of 8 NIH criteria); café-au-lait macules alone are not diagnostic—look for axillary freckling or other features to confirm. [cite: Harrison 21e Ch 471; Nelson 21e; NIH Diagnostic Criteria for NF1 (revised 2021)]