Life and health

Class I glucose-6-phosphate dehydrogenase deficiency

Class I G6PD deficiency, also called severe hemolytic anemia due to G6PD deficiency, is a rare X-linked enzyme disorder with under 10% residual G6PD activity causing chronic non-spherocytic hemolytic anemia, retained as WHO class A in 2022.

Class II aminoacyl-tRNA synthetases

Class II aminoacyl-tRNA synthetases are the ten-member family of tRNA-charging enzymes with an antiparallel beta-sheet catalytic fold, recognized in 1990, that charge 11 of the 22 coded amino acids.

Class II division 2 malocclusion

Class II division 2 malocclusion (ICD-11 DA0E.50): an Angle-classified bite with distal molar occlusion, retroclined upper central incisors, and deep overbite; treatment evidence lacks randomized trials.

CLASS-04 trial

CLASS-04 (NCT02845986) is a Chinese phase 2 single-arm trial of laparoscopic spleen-preserving No. 10 lymph node dissection with total gastrectomy for locally advanced upper-third gastric cancer: 251 patients, 19 centers, 3-year OS 79.1%, DFS 73.1%.

Classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency

Classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency (classic 21-OHD, CYP21A2 deficiency): severe autosomal recessive cortisol and aldosterone deficiency with prenatal androgen excess, salt-wasting or simple virilizing forms, treated with hydrocortisone, fludrocortisone, and newly approved crinecerfont.

Classic eosinophilic pustular folliculitis

Classic eosinophilic pustular folliculitis, also called Ofuji disease, is a rare, sterile, relapsing inflammatory skin disease with eosinophilic infiltration of hair follicles, itchy annular papulopustular plaques on the face and trunk, described by Ofuji in Japan, treated with indomethacin or corticosteroids.

Classic heparin-induced thrombocytopenia

Classic heparin-induced thrombocytopenia (cHIT, typical-onset HIT) is a prothrombotic IgG anti-PF4/heparin adverse drug reaction causing platelet falls 5–10 days after heparin, with 50–70% thrombosis risk, diagnosed by 4Ts score plus immunoassay and functional assays and treated with non-heparin anticoagulants.

Classic Hodgkin lymphoma

Classic Hodgkin lymphoma, also written classical Hodgkin's lymphoma, is a cancer of the immune system marked by Hodgkin and Reed-Sternberg cells, accounting for about 95% of Hodgkin lymphoma cases.

Classic mycosis fungoides

Classic mycosis fungoides (Alibert-Bazin type, ORPHA:2584) is the most common form of cutaneous T-cell lymphoma, progressing slowly from patches on sun-protected skin to plaques and tumors, treated stage-adapted with skin-directed therapy first.

Classical biological control with parasitoid wasps

Classical biological control with parasitoid wasps is the deliberate importation and permanent establishment of exotic parasitoids to suppress introduced pests, aiming for self-sustaining long-term control rather than repeated intervention.

Classical complement pathway

Classical complement pathway: first-described complement activation route, triggered by C1q binding antigen-bound IgM/IgG, forming C4b2a C3 convertase and the membrane attack complex.

Classical cystic fibrosis

Classical (classic, typical) cystic fibrosis, ICD-11 CA25.0, is the severe majority form of CF: CFTR mutations, sweat chloride >60 mmol/l, pancreatic insufficiency, progressive lung disease, distinct from atypical CF.

Classical Ehlers-Danlos syndrome

Classical Ehlers-Danlos syndrome (cEDS), formerly EDS classical type, is a heritable connective tissue disorder with hyperextensible skin, atrophic scarring, and joint hypermobility, caused by COL5A1 or COL5A2 variants affecting type V collagen.

Classical Hodgkin lymphoma, mixed cellularity type

Mixed cellularity classical Hodgkin lymphoma is a subtype making up 20–25% of cases, strongly linked to Epstein–Barr virus, more common in males, and typically presenting at advanced stages.

Classical monocyte

Classical monocyte: the CD14++CD16− (human) / Ly6C-high (mouse) major blood monocyte subset, about 85–90% of monocytes, CCR2-high, phagocytic, and the precursor of inflammatory macrophages; distinct from intermediate and nonclassical monocytes.

Classical non-receptor protein tyrosine phosphatases

Classical non-receptor protein tyrosine phosphatases are the cytosolic members of the phosphotyrosine-specific PTP family, 17 human enzymes such as PTP1B and SHP-2 that oppose tyrosine kinase signaling and are drug targets in diabetes and cancer.

Classical scabies

Classical scabies, also called common or typical scabies, is the standard form of infestation by Sarcoptes scabiei var. hominis with 10–15 mites, nocturnal itch, burrows in finger webs and wrists, and permethrin treatment, distinct from crusted scabies.

Classical swine fever

Classical swine fever, or hog cholera, is a WOAH-listed Pestivirus (Flaviviridae) disease of domestic and wild pigs, the only susceptible hosts. Acute fever with cyanosis, death in 1-2 weeks; endemic in parts of Asia and the Americas, eradicated elsewhere.

Classical-like Ehlers-Danlos syndrome type 1

Classical-like Ehlers-Danlos syndrome type 1 (TNXB-related clEDS) is an autosomal recessive connective tissue disorder from biallelic TNXB variants causing tenascin-X deficiency, with joint hypermobility, hyperextensible skin, easy bruising, and no atrophic scarring.

Classical-like Ehlers-Danlos syndrome type 2

Classical-like Ehlers-Danlos syndrome type 2, also known as AEBP1-related EDS, is a rare autosomal recessive connective tissue disorder causing joint hypermobility and fragile, stretchy skin.

Classification and diagnosis of acute lymphoblastic leukemia

The classification and diagnosis of acute lymphoblastic leukemia covers the 20% blast threshold, obligatory morphology, flow cytometry, and molecular genetics, and the 2022 WHO and ICC genetic subgroups.

Classification of acute myeloid leukemia

Acute myeloid leukemia is classified by the morphologic FAB system, its eight subtypes M0–M7, and the genetic 2022 WHO and ICC classifications that define disease by mutations.

Classification of mental disorders

Classification of mental disorders: organizing mental, behavioral, and neurodevelopmental conditions into diagnostic categories via ICD-11 (WHO, effective 2022) and DSM-5-TR (APA, 2022), with dimensional alternatives RDoC and HiTOP.

Classification of obesity

Classification of obesity: WHO BMI-based classes (obesity ≥30 kg/m², classes I–III), ICD code 5B81, the 2025 Lancet Commission's clinical vs preclinical obesity diagnosis, and the Edmonton Obesity Staging System (EOSS) complications-based staging.

Classification of personality disorders

Classification of personality disorders: how DSM and ICD categorize personality pathology. DSM-5-TR keeps 10 categorical disorders in three clusters; ICD-11 (2022) uses severity plus five trait domains.

Clathrin

Clathrin: eukaryotic coat protein forming triskelia (three heavy, three light chains) that assemble into pentagon-hexagon lattices for coated pits, endocytosis, and trans-Golgi transport; discovered by Pearse after Roth and Porter's 1964 coated-pit description.