1500+ medical terms, defined.

PRPP synthetase catalyzes the formation of phosphoribosyl pyrophosphate (PRPP) from ribose-5-phosphate and ATP, a key committed step in both purine and pyrimidine nucleotide synthesis. Overactivity of this enzyme causes excess purine pro...

Ornithine transcarbamylase (OTC) is a mitochondrial enzyme in the urea cycle that converts carbamoyl phosphate and ornithine into citrulline. OTC deficiency is the most common urea cycle disorder and the only X-linked one, leading to hyp...

Lines of Zahn are alternating pale layers of platelets and fibrin with darker layers of red blood cells seen grossly and microscopically in a thrombus. They indicate a thrombus that formed in flowing blood, and so distinguish an antemort...

Lepidic growth is a pattern in which tumour cells spread along intact alveolar septa without invading stroma, vessels, or pleura, using the alveolar walls as a scaffold. It is characteristic of adenocarcinoma in situ of the lung, formerl...

Schaumann bodies are laminated, concentric calcium and protein inclusions found within the giant cells of the non-caseating granulomas of sarcoidosis. They are characteristic but not pathognomonic, since they also occur in berylliosis an...

Masson bodies are plugs of loose fibromyxoid granulation tissue that fill alveolar ducts and alveoli in organizing pneumonia. They are the histological hallmark of cryptogenic organizing pneumonia, previously called bronchiolitis obliter...

Cholesterol desmolase (side-chain cleavage enzyme, P450scc, encoded by CYP11A1) converts cholesterol to pregnenolone on the inner mitochondrial membrane. It catalyses the rate-limiting and committed step of all steroid hormone synthesis...

Pregnenolone is the first steroid formed from cholesterol by cholesterol desmolase and is the common precursor of every steroid hormone. From pregnenolone the pathway branches to mineralocorticoids, glucocorticoids, androgens, and oestro...

Diplopia is double vision, caused by misalignment of the visual axes so that an image falls on non-corresponding retinal points. Binocular diplopia resolves when either eye is covered and implies an ocular motor problem; monocular diplop...

A bulla is a fluid-filled blister larger than 1 cm in diameter; smaller lesions are vesicles. In the lung the same word describes an air-filled space greater than 1 cm produced by destruction of alveolar walls in emphysema.

Middle East respiratory syndrome is a severe viral pneumonia caused by MERS-CoV, a betacoronavirus with a dromedary camel reservoir. It emerged in Saudi Arabia in 2012 and is notable for a high case fatality rate of roughly 35% with limi...

Lymphadenitis is inflammation of a lymph node, usually producing tender enlargement. Acute lymphadenitis is most often a reactive response to drainage from a nearby infection, while chronic lymphadenitis reflects persistent antigenic sti...

Etiology is the cause or set of causes of a disease. It is distinguished from pathogenesis, which is the mechanism by which that cause produces the disease.

Woolsorter's disease is inhalational anthrax, caused by inhaling Bacillus anthracis spores from contaminated animal hides, wool, or goat hair. It classically produces haemorrhagic mediastinitis with a widened mediastinum on chest radiogr...

Hyaluronidase is an enzyme that degrades hyaluronic acid in connective tissue ground substance. Bacteria such as Streptococcus pyogenes and Staphylococcus aureus secrete it as a spreading factor that allows infection to dissect rapidly t...

Sydenham chorea is a late neurological manifestation of acute rheumatic fever, causing involuntary, rapid, non-rhythmic movements plus emotional lability and hypotonia. It is a major Jones criterion and may appear months after the strept...

Erythema marginatum is the evanescent, non-pruritic rash of acute rheumatic fever, appearing as pink macules with serpiginous, raised borders and central clearing on the trunk and proximal limbs. It is a major Jones criterion but is unco...

Warthin-Finkeldey cells are multinucleated giant cells with numerous nuclei and eosinophilic inclusions found in lymphoid tissue during measles infection. They are considered pathognomonic for measles and appear in tonsils, lymph nodes,...

Subacute sclerosing panencephalitis is a rare, uniformly fatal degenerative brain disease occurring years after measles infection, caused by persistence of a defective measles virus in the central nervous system. It presents with persona...

Koplik spots are small blue-white papules on an erythematous base on the buccal mucosa, classically opposite the second molars, and are pathognomonic for measles. They appear one to two days before the skin rash and fade as it erupts.

Coryza is acute inflammation of the nasal mucosa producing profuse nasal discharge and congestion, in plain terms a runny, stuffy nose. It is one of the three Cs of the measles prodrome alongside cough and conjunctivitis.

Rubeola is another name for measles, an acute, highly contagious paramyxovirus infection with fever, the three Cs, Koplik spots, and a descending maculopapular rash. It must be distinguished from rubella, which is a separate, far milder...

Albuminocytologic dissociation is a cerebrospinal fluid pattern of raised protein with a normal or near-normal white cell count. It is the classic CSF finding in Guillain-Barre syndrome and in chronic inflammatory demyelinating polyneuro...

Orthomyxoviridae is the family of the influenza viruses, enveloped viruses with a segmented, negative-sense, single-stranded RNA genome. The segmented genome is what allows genetic reassortment and therefore antigenic shift and pandemics.

Neuraminidase (sialidase) is an influenza surface glycoprotein that cleaves sialic acid residues from host cell glycoproteins, releasing newly assembled virions from the infected cell. It is the target of oseltamivir and zanamivir.

Reoviridae are non-enveloped viruses with a segmented, double-stranded RNA genome and a distinctive double protein capsid. The medically important members are rotavirus, the leading cause of severe paediatric gastroenteritis, and Colorad...

Sialic acid is a family of nine-carbon acidic sugars, chiefly N-acetylneuraminic acid, that cap the ends of glycoprotein and glycolipid chains on cell surfaces. It serves as the influenza receptor and, on host cells, as a self-marker tha...

Varicella-zoster virus is human herpesvirus 3, an enveloped double-stranded DNA virus causing chickenpox on primary infection and shingles on reactivation. After primary infection it establishes lifelong latency in dorsal root and crania...

Rimantadine is an antiviral that blocks the influenza A M2 proton channel, preventing viral uncoating. It is no longer recommended because circulating influenza A strains are almost universally resistant.

Amantadine blocks the influenza A M2 proton channel to prevent viral uncoating, and separately increases dopamine release and blocks NMDA receptors. Its antiviral use is obsolete because of resistance, but it remains useful in Parkinson...

Purulent means containing or producing pus, an exudate of dead neutrophils, liquefied tissue debris, and often bacteria. Purulent inflammation is the hallmark of pyogenic bacterial infection.

CD4 is a surface glycoprotein on helper T cells that binds MHC class II and stabilizes antigen presentation. It is also the primary receptor used by HIV to enter cells, which is why the CD4 count tracks the stage of HIV disease.

Human immunodeficiency virus is an enveloped retrovirus with two copies of positive-sense single-stranded RNA and reverse transcriptase, which progressively destroys CD4-positive T cells. Untreated infection advances to AIDS, defined by...

gp120 is the HIV envelope surface glycoprotein that binds CD4 and then a chemokine coreceptor, CCR5 or CXCR4, to initiate entry. Its extreme variability and glycan shielding are central obstacles to an effective HIV vaccine.

The S (spike) glycoprotein is the surface protein of coronaviruses that binds the host receptor and mediates membrane fusion, giving the virion its crown-like appearance on electron microscopy. It determines host range and tissue tropism...

MERS-CoV is the betacoronavirus that causes Middle East respiratory syndrome, entering cells through dipeptidyl peptidase 4 rather than ACE2. Dromedary camels are the reservoir, and the case fatality rate is roughly 35%.

Severe acute respiratory syndrome is an atypical pneumonia caused by SARS-CoV, a betacoronavirus that emerged in Guangdong, China in 2002 and spread internationally before being contained in 2003. It binds ACE2, had a case fatality rate...

A bicornuate uterus is a Mullerian duct fusion defect producing a heart-shaped uterus with two horns and a single cervix. It is associated with recurrent pregnancy loss, preterm birth, and malpresentation.

McBurney point lies one-third of the way along a line from the right anterior superior iliac spine to the umbilicus, and marks the surface projection of the base of the appendix. Maximal tenderness at this point (McBurney sign) is a clas...

McBurney point is the surface landmark for the base of the appendix, located one-third of the way from the right anterior superior iliac spine to the umbilicus. Tenderness here is the classic sign of acute appendicitis.

Enriched breads are refined grain products to which iron and the B vitamins thiamine, riboflavin, niacin, and folic acid have been added back after milling removes them. Mandatory folic acid fortification has substantially reduced neural...

Gummas are the granulomatous, necrotic lesions of tertiary syphilis, occurring in skin, bone, and viscera years after untreated infection. They are firm, rubbery masses with central coagulative necrosis and represent a delayed hypersensi...

An aplastic crisis is an abrupt, transient arrest of erythrocyte production causing a sudden fall in haemoglobin with a low reticulocyte count. It is classically precipitated by parvovirus B19 in patients who depend on rapid red cell tur...

The whiff test adds 10% potassium hydroxide to vaginal discharge and is positive when a fishy amine odour is released. A positive whiff test is one of the Amsel criteria for bacterial vaginosis.

Chlamydophila psittaci is an obligate intracellular bacterium acquired from birds, especially parrots and parakeets, that causes psittacosis, an atypical pneumonia. Exposure history to birds is the diagnostic clue.

A staccato cough is a series of short, choppy, individually separated coughs without an intervening whoop. In an afebrile infant of one to three months with tachypnoea and conjunctivitis it strongly suggests Chlamydia trachomatis pneumonia.

The Weil-Felix test is a historical serological assay for rickettsial infection that relies on cross-reaction between rickettsial antibodies and Proteus vulgaris O antigens. It is insensitive and non-specific and has been replaced by ind...

Proteus vulgaris is a gram-negative, urease-producing enteric rod that swarms across agar in concentric waves and causes urinary tract infections. Its urease alkalinizes urine and promotes struvite staghorn calculi.

Rickettsia are obligate intracellular gram-negative coccobacilli transmitted by arthropod vectors that infect vascular endothelium, producing vasculitis, rash, fever, and headache. Doxycycline is the treatment for all rickettsial disease...

Haemophilus ducreyi is a gram-negative coccobacillus that causes chancroid, a sexually transmitted infection producing painful genital ulcers with painful suppurative inguinal lymphadenopathy. The mnemonic is that you 'do cry' with ducreyi.