Question 27 from the second paper of 2019 explored high altitude pulmonary oedema (HAPE) and high altitude cerebral oedema (HACE). As there was nowhere else to put this topic, it went to Misc. The unprepared trainees, confronted with HACE and HAPE, generally did very poorly (12.3% pass rate). Given that it could not possibly be viewed as a critically important knowledge territory, one can expect that it will never appear in the papers again. In case it does, the trainees will be prepared.
The single best (free) article covering this territory is probably Mehta et al (2011), from the Medical Journal of the Armed Forces of India.
The definition relies on the physiological changes which occur at an altitude of around 2700 meters above mean sea level. These changes are mainly related to the changes in alveolar partial oxygen pressure (about 60mmHg at this altitude), which is in turn related to atmospheric pressure (around 540 mmHg). As one ascends higher, the alveolar oxygen tension decreases. The "death zone" is said to be above 8,000m, where atmospheric pressure is around 260, and prolonged survival without supplemental oxygen is impossible.
Mehta, S. R., A. Chawla, and A. S. Kashyap. "Acute mountain sickness, high altitude cerebral oedema, high altitude pulmonary oedema: The current concepts." Medical journal, Armed Forces India 64.2 (2008): 149.
Basnyat, Buddha, and David R. Murdoch. "High-altitude illness." The Lancet 361.9373 (2003): 1967-1974.
Hackett, Peter H., and Robert C. Roach. "High-altitude illness." New England journal of medicine 345.2 (2001): 107-114.
Bhagi, Shuchi, Swati Srivastava, and Shashi Bala Singh. "High-altitude pulmonary edema." Journal of occupational health (2014): 13-0256.
Basnyat, Buddha. "High altitude cerebral and pulmonary edema." Travel medicine and infectious disease 3.4 (2005): 199-211.
Stuber, Thomas, and Yves Allemann. "High altitude illness-pathogenesis and treatment." SCHWEIZERISCHE ZEITSCHRIFT FUR SPORTMEDIZIN UND SPORTTRAUMATOLOGIE 53.2 (2005): 88.