17. Possibility Two

Complete story — includes the ending

While helping Joan learn deductive skills, Sherlock is approached by Gerald Lydon, a wealthy man suffering from an incurable disease that is destroying his mind. The disease is genetic. However, there is no family history of the disease, so Lydon is certain that someone gave it to him somehow. Unconvinced, Sherlock refuses the case. After Lydon kills his driver, however, Sherlock takes the case and learns that Lydon's claim is theoretically possible. He receives an email from Dr. Natasha Kademan providing details of how it was done, but before they can meet, she turns up dead. All the evidence, including DNA results from blood found at the crime scene, point to the killer being an ex-con with no connection to Sherlock's case. However, the blood was actually manufactured to pass the DNA test. The killer turns out to be Kademan's husband, who mistakenly believed that she was having an affair with someone named Lincoln Dunwoody. The wealthy Lincoln and Dunwoody families each established a foundation to conduct research on Lydon's disease because a member of each family contracted the disease. A scientist suffering from the disease poisoned all three wealthy individuals to obtain funding for research for a cure.