The following fictional case is intended as a learning tool within the Pathology Competencies for Medical Education (PCME), a set of national standards for teaching pathology. These are divided into three basic competencies: Disease Mechanisms and Processes, Organ System Pathology, and Diagnostic Medicine and Therapeutic Pathology. For additional information, and a full list of learning objectives for all three competencies, see http://journals.sagepub.com/doi/10.1177/2374289517715040.¹.

Urinary symptoms are one of the most common reasons for emergency visits in females of pediatric age group and can be associated with various conditions like infections (most common), sexual trauma and rarely neoplastic processes. Here, we report a case of a 7-year-old female who presented in the emergency multiple times with the complaints of urinary symptoms and vaginal pain and was empirically treated with antibiotics and antifungals without symptomatic improvement. Her blood tests, physical examination during this time remained unrevealing. She was then transferred to our institution on her third emergency visit for further evaluation. On imaging studies, she was noted to have expansile lesions on her vertebral body at the L4 and T6 levels with compressive myelopathy with multiple bone and soft tissue lesions throughout her lower extremities. Patient developed saddle anesthesia requiring emergent decompression and biopsy of the epidural mass with the final pathology coming back as B-lymphoblastic leukemia/lymphoma. B-ALL/B-LBL is the most common pediatric hematologic malignancy and usually presents with fever, hepatosplenomegaly, lymphadenopathy, bone pain and bleeding. Occasionally, atypical presentations like bone and joint pain, osteoporosis, palpable paravertebral mass have been described. However, this is the first case report to describe a very unusual and unfamiliar presentation of this disease causing significant diagnostic difficulty resulting in delayed treatment. This case report can aid as a reminder that unusual pain or any nonspecific manifestations in pediatric patients, refractory to common treatment should be investigated with extreme diligence not to miss this neoplastic process.

Classical Hodgkin lymphoma (CHL) is recognized as a B-cell neoplasm arising from germinal center or postgerminal center B-cells. The hallmark of CHL is the presence of CD30 (+) Hodgkin and Reed-Sternberg (HRS) cells with dim expression of PAX5. Nearly all of the HRS cells are positive for PAX5. However, a small minority of HRS cells may lack PAX5 expression, which can cause a diagnostic dilemma. Herein we describe two cases of PAX5-negative CHL and review of the English literature on this very rare entity. It is crucial to be aware of this phenomenon, which in some cases may lead to misdiagnosis and may ultimately adversely affect patient's management.

Increased interest in the opportunities provided by artificial intelligence and machine learning has spawned a new field of health-care research. The new tools under development are targeting many aspects of medical practice, including changes to the practice of pathology and laboratory medicine. Optimal design in these powerful tools requires cross-disciplinary literacy, including basic knowledge and understanding of critical concepts that have traditionally been unfamiliar to pathologists and laboratorians. This review provides definitions and basic knowledge of machine learning categories (supervised, unsupervised, and reinforcement learning), introduces the underlying concept of the bias-variance trade-off as an important foundation in supervised machine learning, and discusses approaches to the supervised machine learning study design along with an overview and description of common supervised machine learning algorithms (linear regression, logistic regression, Naive Bayes, k-nearest neighbor, support vector machine, random forest, convolutional neural networks).