By Magdalena Mis LONDON, April 21 (Thomson Reuters Foundation) - Health services in Yemen are on the brink of collapse amid critical shortages of life-saving medicines, oxygen and fuel, the World Health Organization (WHO) said on Tuesday. Prices of essential medicines have increased by more than 300 percent, and the shortage of water has increased the risk of diarrhea and other diseases and is affecting basic hygiene in hospitals and clinics, the Geneva-based organization said.