An 8‑month‑old male dog is brought to a veterinarian because…

An 8‑month‑old male dog is brought to a veterinarian because of a 1‑month history of progressive lethargy, decreased appetite, and shifting limb lameness. Physical exam reveals pain on palpation of multiple long bones and a mild fever of 39.8 °C; (rr, 38.0–39.3 °C). Complete blood count shows a hematocrit of 25% (rr, 37–55%), indicating anemia, and a platelet count of 80 × 10^9/L (rr, 200–500 × 10^9/L), indicating thrombocytopenia. Serum biochemistry is otherwise unremarkable. Radiographs demonstrate multiple aggressive lytic lesions involving the metaphyses of long bones, ribs, and vertebrae. Cytology from bone aspirates reveals large atypical round cells with a high nuclear‑to‑cytoplasmic ratio and prominent nucleoli. Flow cytometry shows a uniform population of lymphoid cells that are positive for CD20 and PAX5 and negative for CD3. Which of the following diagnoses best explains this dog’s clinical, radiographic, and immunophenotypic findings?

A 17-year-old male presents with fever, headache, photophobi…

A 17-year-old male presents with fever, headache, photophobia, and neck stiffness for 1 day. Cerebrospinal fluid analysis is consistent with bacterial meningitis, and blood cultures grow Gram-negative diplococci. His medical history is notable for two prior episodes of meningitis during early adolescence. Between infections, he has been otherwise healthy with normal growth and development. Labs show normal immunoglobulin class levels [Serum IgG: 1,150 mg/dL (reference: 700–1,600 mg/dL), Serum IgA: 180 mg/dL (reference: 70–400 mg/dL), Serum IgM: 120 mg/dL (reference: 40–230 mg/dL),] and normal C3 and C4 levels [C3: 110 mg/dL (reference: 90–180 mg/dL), C4: 28 mg/dL (reference: 10–40 mg/dL)] but low CH50 (Total hemolytic complement (CH50):