Is cognitive dysfunction an early stage of alzheimers?

Possibly.: Alzheimer’s disease usually has unrecognized memory loss initially. Then things like lack of organization for daily living, like keeping track of finances get noticed by others (friends, family). Patients have disorientation, aphasia (language problems), apraxia (cannot do learned movements), alexia (unable to read). A person's ability to recognize people can be affected but usually later.

Many reasons: Mild cognitive impairment is often the initial manifestation of Alzheimer's but cognitive problems can be seen with vitamin deficiencies, thyroid problems, HIV and syphilis infections, and medication adverse effects. Alcohol and recreational drugs can also do this.