Every morning, Sergio Castro crisscrosses this city to treat the intimate struggles behind its closed doors. Past a black metal gate, Diego Raúl López Sánchez lay on a bed in a concrete room. A motorcycle crash left him paralyzed from the neck down a few months ago, and bedsores have branded his emaciated body. Mr. Castro cleaned and dressed the broken skin as he murmured softly to his patient. He offered advice to Mr. López's wife, who seemed numb with despair at her husband's new reality. He would return the next day.