MAC

**1. Minimum Alveolar Concentration (Pharmacology/Anesthesia)** - Definition: the alveolar concentration of an inhaled anesthetic at 1 atm that prevents movement in response to a surgical stimulus in 50% of patients. - MAC is inversely proportional to potency and directly related to lipid solubility: higher lipid solubility means lower MAC and higher potency (Meyer-Overton). Examples: halothane (MAC ~0.75%) is more potent than nitrous oxide (MAC ~105%). - Blood:gas partition coefficient is inversely related to speed of induction and recovery. Low solubility (N2O, desflurane, sevoflurane) gives fast onset and offset. - MAC increases with: hyperthermia, chronic alcohol use, young age (peaks in infants), and hypernatremia/CNS stimulants (acute amphetamine). - MAC decreases with: old age, pregnancy, hypothermia, hypotension, hyponatremia, acute alcohol intoxication, opioids/benzodiazepines/other sedatives, and α2-agonists. - Adverse effects: halothane causes hepatotoxicity ("halothane hepatitis"); all volatile agents and succinylcholine can trigger malignant hyperthermia (RYR1 mutation, treat with dantrolene); methoxyflurane causes nephrotoxicity; N2O can expand air-filled spaces and inactivate B12 (methionine synthase). - Related term: Monitored Anesthesia Care (MAC) is sedation with anesthesiologist monitoring and no general anesthesia.

**2. Membrane Attack Complex (Immunology)** - Formed by C5b, C6, C7, C8, and multiple C9 molecules (C5b-9), the terminal common pathway of all three complement pathways (classical, alternative, lectin). - It creates pores in cell membranes, causing osmotic lysis. It is especially important for killing encapsulated *Neisseria*. - Deficiency of C5–C9 leads to recurrent Neisseria infections (meningococcemia, disseminated gonococcal infection). - Deficiency of DAF (CD55) and CD59 (which normally block MAC formation on host cells) causes paroxysmal nocturnal hemoglobinuria (PNH): GPI-anchor defect (PIGA mutation), complement-mediated intravascular hemolysis, hemoglobinuria, and thrombosis. Treat with eculizumab (anti-C5). Eculizumab raises the risk of Neisseria infection, so give meningococcal vaccination. - C1 esterase inhibitor deficiency causes hereditary angioedema (not MAC, but a frequently paired complement question).

**3. Mycobacterium avium Complex (Microbiology/Infectious Disease)** - *M. avium* and *M. intracellulare* are non-tuberculous mycobacteria (acid-fast, found in soil and water). - Causes disseminated disease in AIDS patients with CD4 <50/µL: fever, night sweats, weight loss, diarrhea or malabsorption, hepatosplenomegaly, lymphadenopathy, and anemia. - In immunocompetent patients with structural lung disease (COPD, bronchiectasis), it causes pulmonary disease. Hot tub lung is a hypersensitivity pneumonitis linked to MAC. Lady Windermere syndrome is middle lobe/lingular bronchiectasis in older women. - Prophylaxis: azithromycin when CD4 <50, though this is no longer routine if ART is started right away. - Treatment: macrolide (clarithromycin or azithromycin) + ethambutol ± rifabutin.

**4. Mitral Annular Calcification (Cardiology)** - Degenerative calcification of the mitral valve annulus, seen in older adults, women, and patients with chronic kidney disease or hypertension, and associated with atherosclerosis. - Usually asymptomatic. It can cause mitral regurgitation (more common) or stenosis, conduction abnormalities, and embolic events. - Echocardiography shows an echogenic ring at the base of the mitral leaflets.

**Test strategy:** Identify the stem. Anesthetic potency or "lowest concentration preventing movement" points to alveolar concentration. Neisseria infections or a terminal complement deficiency point to the membrane attack complex. Advanced HIV with CD4 <50 and an acid-fast organism points to *Mycobacterium avium* complex.